I have to cut a 2.5" hole into my 2x10 floor joists (actually measures closer to 8.5") to run pvc venting. Was wondering if this is acceptable or will it be too much and weaken the structural integrity?

How many joists, what location in the span of the joists?

Look, I will tell you right now that it isn't advisable to cut through joists if you can help it in that size, but it depends where it is, and how many. There are codes about this as well.

So, a bit more detail would be helpful.

If you can run that vent to a wall side, where perhaps it can go through wall studs before going up, that would be your best bet. If you are going to bore a hole in the joist at that size, doing it as little as possible is the best plan, AND the method is where it is in the span, and in the middle of the joist.

Now that I have added this, send a pic or give more information. The plumbing guys can help you plan the best run for a vent.

From an old code book but presumably still correct.