I've got a 3" PVC joint where the end of the pipe went in 1" instead of the full seated depth of 1-1/2" thanks to not having enough leverage to twist and push the joint together before it set. Does this joint need to be cut out and replaced, or is it good enough?

Technically speaking, it is wrong. Yes, the end of pipe should have been inserted all the way in. So my answer would be : re-do it.

Is it in exposed location or in the wall / ceiling ?

If this was drain pipe, I would definitely say cut it and replace it. But since it is vent - and hopefully in exposed location - I will say leave it the way it is.

It is in a horizontal section of the drain plumbing under the 2nd floor. It will be concealed in a soffit. Sounds like the right thing to do is cut it out and do it again. Any hints on getting enough leverage to get it to seat?

Bad news: if it is Sewer Drain and in enclosed Horizontal location - you will have to re-do it.

1. Cut out 16" of old pipe.
2. Cut 15 1/2" of new PVC pipe.
3. Slip on Fernco shielded coupling on one end of this new pipe and glue PVC coupling on the other end.
4. Glue end with coupling onto existing pipe.
5. Go to the other end new of pipe: Slip Fernco coupling over to reach the other cut end of existing pipe.
6. Tighten 2 s.s. screws
7. Test.

That should do... Back to you... Milo