I noticed my kitchen sink was draining slowly, so I used a plunger and turned on disposal and wated drained, but when I turned disposal off wated came back up sink. Did this twice and then water started coming out wall about sink. Torn sheet rock down and found pipe was not even sealed and this is a vent pipe so water should not even be coming from this. Have removed sheet rock and plywood to expose plumbing and found that the water is backing up from the drain to the vent pipe. If I seal this pipe will it cause the air pressure to push the water down the drain instead of backing in to the vent?

If I seal this pipe will it cause the air pressure to push the water down the drain instead of backing in to the vent?No! It will simply back up in your sink. It's illegal to terminate a vent in a wall. Vents should be run out the roof or tied back to another roof vent in the attic. Have this looked after at once!! You have a direct connection between the city sewer and your home that's allowing sewer gas to enter.
Now, on to your blockage. Send a snake down the open vent in your wall. Put out at least 16 feet to get out to the main.
And get that vent fixed. Pronto! Good luck, Tom