I have a double sink vanity, that has a clog somewhere downstream of what I assume to be the sani-tee, when I tried snaking it (starting from the trap), the snake just comes out the other trap. I also tried plunging one sink with a cap on the other trap, but I just hear water slushing around(in the vent pipe), but nothing moving. So my options are to either hop up on the roof and snake the vent pipe, or add a cleanout/tee to the ventpipe in the attic, I would prefer the attic idea since it is safer and easy to do again if I have to, just wanted to check to make sure its OK to do.

It's OK.

YUP! As Joypulv said, no issue with cutting a cleanout into a vent pipe.

However, before you go that route you may want to look into renting a snake with a DROP HEAD tip on it... made for issues like yours. Here, the head on the snake swivels in a manner that keeps it in a "dropped" position so that when you hit the sanitary tee fitting the snake tries to drop into the drain line via gravity. All you should need to do is to measure out the distance and work the snake slowly in the area as you measured out. It doesn't work every time, but it will work eventually and force the snake to go down the drain line, OK?

Questions? Let us know...

Mark