It's not the vent.
You may have something in the trapway, it could be anything. Closet augers sold in hardware stores are close to useless at pushing things out, they are too small and can easily pass right by. Hot water? Does hot water get rid of a plastic shampoo bottle, or a toy car?
There are times, when it's better to just replace the bowl, if you can't clear the trapway. What you have costs maybe $75.00 for tank and bowl. They do plug up a lot, but then do allow the builder to sell the home. If you want a toilet that works without plunging a lot, then you can shop for a good aftermarket bowl and move on with your life. My daughter lived with three pluging toilets for two years before she admitted that her new home had "builder grade" poor performing toilets. You would have thought that plugging three toilets on their first day in the home would have been a clue.
We had a similar job we went out to, the first plumber told the homeowner that they had a defective bowl.
Jamie went out with his auger, and decided to pull the bowl.
This is what he found.
The first plumber had run his auger through, but it went right by Mr GI Joe.
I've always believed that if a bowl is having a problem, you should at least pull it out, and look in the lower part of the trapway. Mirrors help too.