what aadame said, but going into more depth.
what recording software are you using? experiencing lag while recording isn't going to happen if you use the right tools, if possible, of course.

having a nvidia gpu allows you (and you really should) to get geforce experience. it's ran directly by your gpu and has no visible impact on games. possibly 0,1fps slower, but yeah. it also creates mp4 files with great quality, and pretty much no size, which is good if you'd start to run out of memory.
but all that is only possibly with a nvidia gpu. else, i can only recommend fraps, but that has a huge impact on your computer when recorded (and even open).

also, getting any earlier version of vegas should work great. i personally prefer ae over sv, but i mean i do know ae needs far better components to work with than sv.
you can still do pretty neat stuff in the older versions of sv. i mean just look at aadames videos.