Helping a friend out. I want to double check before I suggest my idea. Doing rough plumbing before basement concrete in this area we broke out. Tied into the street sewer main with 4". Doing a 45 right after, then using a 4" San-Wye to catch the toilet with 3" out of the Wye. Then right after that Wye is another facing the other side to catch the lavatory. The toilet is going to be right next to the lavatory, and in terms of pipe, the sink will be tying in just above the toilet Wye. Running 2" for the lavatory. I'm planning on the toilet being vented by the sink drain. As there will be a San tee right after the lavatory trap in the vertical position to drain down and vent up. This makes the vent within 6' of the toilet. Does this sound full proof? Theres not much room to vent the toilet differently.

Go for it.

Where does the vent go after it leaves the lavatory?

within reason.. i doesn't matter how far the san tee behind the sink is from the toilet drain. the critical measure is how far the 3 x 2 "Y" is from the toilet.
put a cleanout in the 2 inch pipe somewhere and keep it full size 2 inch all the way thru the roof.

A 2" cleanout is somewhat useless on a 3" drain line, wouldn't you thing?

probably more useful that the 2016 red sox..
obviously the 2 inch cleanout is required at the base of the sink drain/toilet wet vent..

Dang Red Sox.