I recently had a bathroom remodeled. A new tub and sink were installed but the existing water drain in the middle of the bathroom floor was removed. The new sink is the type that has no water drainage if overfilled. The problem is that now the sink drains very, slowly with a lot of bubbling. The plumber said he needs to reopen the new tiled floor and install a drain again. Can this be avoided with a studor redi-vent or better a trap vent.

The second bathroom had two floor drains one in the shower and the other in the middle of the bathroom as well, which was removed after the remodel. Prior, there use to be bad odors. The new sink or toilet has not been installed yet, so I don't know if I would have other flushing trouble, but should I consider also a studor vent here or reopen that water drain again.
Thanks for the help.

Or is this gurgling, slow draining just fixed with a battha bath vesselsink umbrella type drain as shown in other sections?

Try this, fill the vessel about 4 inches of water and with the plunger out, how well does it drain? Also try this, fill it with no plunger and with the water running, put a straw inside the drain above the water but also about 3 inches below the drain at the vessel bottom. Does this change anything?
What size drain does the plumber say is below the floor? Did he replace any of that pipe? Did he eliminate any vents? Is the trap below the vessel or below the floor?
Don't be so quick to open the tile.

I can see no advantage to reinstalling a floor drain in the floor. It certainly won't help your vessel sink drain better. Check back on the vessel sink problems that have been questioned and answered to fiind your solution. A AAV won't help this problem. A umbrella type drain will help with your drainage problem,(if you haven't already installed one). Good luck, Tom