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:
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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), 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 6:2). 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.
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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 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 2:49). 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 6:5). Our shepherds are "greedy dogs which can never have enough" (Isaiah 56:11) and have become hirelings (John 10:12-13). 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 3:17). May the Lord save us from our own idolatry and covetousness.
