Please distinguish the terms "vow", "oath", "swear" and "pledge." Some of these are interchangeable and some are referred to in the bible as totally different from each other. I'm specifically interested in what Jesus meant in Matthew 5:33-37 [33] Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: [34] But I say unto you, Swear not at all; neither by heaven; for it is God's throne: [35] Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. [36] Neither shalt thou swear by thy head, because thou canst not make one hair white or black. [37] But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
See All... and how that applies to nowadays "pledging" for a building fund or other church projects.

I will start from the general definition of the terms and then get to your particular situation. 

The four terms you mention--vow, oath, swear, and pledge--are referring to three very different situations. Only three, because swear and oath go together. In the Bible, to swear is to make an oath and an oath is what someone swears. Consider these meanings:

• Oath, Swear: an oath is a solemn declaration, usually based on an appeal to God or to some revered person or object (Matthew 5:34-36 [34] But I say unto you, Swear not at all; neither by heaven; for it is God's throne: [35] Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. [36] Neither shalt thou swear by thy head, because thou canst not make one hair white or black.
  See All...), that someone will do some particular thing--like speak the truth, perform a particular act, keep a promise, etc. To swear is to make the oath. To forswear is to swear falsely.
• Vow: in scripture, vow is both an noun and a verb. One can "vow a vow" (Numbers 6:2Speak unto the children of Israel, and say unto them, When either man or woman shall separate themselves to vow a vow of a Nazarite, to separate themselves unto the LORD:
  See All...). A vow is a solemn promise made to the Lord. It usually involves dedicating oneself or some possession to an act, service, or way of life.
• Pledge: often used today as a verb meaning to make a promise to give something. But in the Bible, pledge is consistently used as a noun. Normally, it refers to what we might today call collateral. A pledge was an object given as a guarantee that the promised act or object or money will follow. It was like the earnest money in a real estate purchase or the collateral on a loan. Though it came to mean the making of a promise, it did not originally have this meaning.

Your question specifically refers to the taking of pledges for a building fund. As you can see, the biblical use of the word does not perfectly match with the modern usage where it means a promise. It is not a vow because a vow is a promise made to God--a modern financial pledge is not made to God. Neither is it specifically an oath. No one swears on their Bible that they will give this money.

In truth, a pledge as used today to raise money for buildings is a promise. Making a pledge (as used today) is not the same thing as making an oath. Therefore, the warning of Matthew 5:33-37 [33] Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: [34] But I say unto you, Swear not at all; neither by heaven; for it is God's throne: [35] Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. [36] Neither shalt thou swear by thy head, because thou canst not make one hair white or black. [37] But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
See All... does not refer to this practice. However, this is not to say that God is well-pleased with the way churches raise money today. In 2 Corinthians 8-9, Paul put the pressure on the Corinthians when raising money for the saints in Judea. Today, we put even more pressure on for the building of ever larger and grander buildings. And our methods? They are not the methods of the New Testament. No! We have taken our money-raising methods from the world.

I have seen seminars advertised that had sessions on how to get more money from people, how to get larger pledges, even how to influence the elderly to put the church in their will. Instead of simply praying for God to touch hearts and preaching the need, churches have been so influenced by the world that the two cannot even be distinguished. The high pressure sells techniques are carefully planned and orchestrated. Special fund-raisers (with amazingly high fees) are brought in to squeeze more money out of the people. All for bigger buildings.

Does it work? It depends on how you look at it. Most often, the money is raised and the buildings are built. But too often, the Spirit of the Lord is quenched. Many times, the missionaries are denied needed funds and the ministries of the church suffer. Sometimes, the angels write Ichabod across the front of the packed church building, and, as the name means, the glory of the Lord is departed.

I know I am in the minority on this one. How can churches have crowds without Family Life Centers and how can they have those Family Life Centers without high-octane money-raising projects? Church is, after all, big business. Didn't Jesus say that He must was about big business? Well, perhaps He did not exactly say that. The actual quote is, "wist ye not that I must be about my Father's business?" (Luke 2:49And he said unto them, How is it that ye sought me? wist ye not that I must be about my Father's business?
See All...). That must be where we have missed out. We think that the Father's business is big business.

The Lord must be truly grieved at the churches of today. We think that gain is godliness (1 Timothy 6:5Perverse disputings of men of corrupt minds, and destitute of the truth, supposing that gain is godliness: from such withdraw thyself.
See All...). Our shepherds are "greedy dogs which can never have enough" (Isaiah 56:11Yea, they are greedy dogs which can never have enough, and they are shepherds that cannot understand: they all look to their own way, every one for his gain, from his quarter.
See All...) and have become hirelings (John 10:12-13 [12] But he that is an hireling, and not the shepherd, whose own the sheep are not, seeth the wolf coming, and leaveth the sheep, and fleeth: and the wolf catcheth them, and scattereth the sheep. [13] The hireling fleeth, because he is an hireling, and careth not for the sheep.
See All...). We are building physical, earthly kingdoms that will burn with everything else some day soon. The Laodicean church says, "I am rich, and increased with goods, and have need of nothing; and knowest not that thou art wretched, and miserable, and poor, and blind, and naked" (Revelation 3:17Because thou sayest, I am rich, and increased with goods, and have need of nothing; and knowest not that thou art wretched, and miserable, and poor, and blind, and naked:
See All...). May the Lord save us from our own idolatry and covetousness.