I purchased Vegas Pro 21 a few months ago. Can my son living in the same house install it on his computer without effecting my install on my own computer?
Also, will older edit files from Vegas Pro 11 ( *.veg ) open and run normally on Vegas Pro 21?
If you did the purchase vs subscription it works on two computers so you can install it on both.
You can open .veg files from 11 in all newer versions of VEGAS. If there are Fx (plugins, etc.) that you had with 11 but are not installed with or work with 21 it's possible the project won't look right or won't open but in general that doesn't happen.
Enjoy 21, things have come a long way since 11. If you have problems with it feel free to ask here. I settled on using build 208 of 21 as I have an NVIDIA GPU and got some graphical glitches with the final builds of 21. They are all downloadable from here. https://www.vegascreativesoftware.info/us/forum/faq-where-can-i-download-vegas-pro-and-other-vegas-software--104782/
Please note also this older but related thread https://www.vegascreativesoftware.info/us/forum/installing-vegas-pro-on-a-second-computer--128284/