I always seem to accumulate vegetable scraps faster than I do bones, so would it be possible to create a vegetable stock in a pressure cooker, to then create a chicken /beef stock at a later date when the bones are available? Specifically, would there be a considerable loss in flavor, or would there be a better way to go about this?
I'm not certain what you are asking? Are you wanting to start a vegetable stock and use it as a base for the bone stock at a later time ?
IMO that would work.
Do a vegetable stock when you have enough vegetable scraps, portion it and freeze it.
When you have enough bones, do the stock using the vegetable stock use made earlier.
As an aside, your local grocer/meat department/butcher will put scraps together for you (beef bones, chicken necks, etc) from whatever they are cutting that day. It usually sells for $1/pound (in my area). Just call early! Eliminates the long wait to get enough of what you need/want for your stock.