I can’t help you with these STP specifics.
What I can say, though, is that it’s important with whatever software tool you take up to take a little time to find and work out what you’ve got, and then develop a workflow that accomplishes your goals, but also works with the “grain” of the tool, rather than trying to force it to do things in the way of some other tool you used before, or how you think it “ought” to work.
If the subclip handling in FCP doesn’t suit you, you could render / export new clips as you go. Personally, I find capturing long clips and then sub-clipping them always puts extra load on the computer in editing and slows down performance and responsiveness unnecessarily.
FCP has much to offer – as Vegas probably does too, though I’ve never used it. I’ve no doubt that there’s a way to do what you need using FCP – so many users, so many programmes completed with this tool suggest that it can get the job. Whether you can find a way to accommodate your specific workflow needs, only you can decide.