It will make almost no difference to increase to 16GB of memory. If you have 8GB of memory, the first thing to think about is how good is your video card and processor. Can you list the specs for your friends PC?
It will make a difference. I run 16 GB with Vegas 11 and it caches up the lot in RAM in a matter of minutes. This is especially true when working with HD files. Vegas Platinum is also 32 bit while rival 64 bit offerings are even more RAM intensive. Abode software loaded up 10 GB of RAM at one point and the only program I ever saw do that was RAM drive.
Wamphryi is correct. You're not the typical gamer or alike. You will definitely see a difference in Sony Vegas and switching between apps while that's open. And given the reasonable cost in RAM these days, it's worth the few bucks...
sony vegas is an demanding software but my brother edit's a lot of vids on powerdirector 10 with 8gb ram.i am not saying that there won't be any difference,but it's going to be small.all long as your cpu is strong,vegas won't need over 8gb.
If your RAM usage is hitting 70% then personally I would add more RAM. The application(s) unless poorly written will leave some RAM. The tell tale sign is IF you have any 'FREE' RAM listed as shown below and IF you don't then the application(s) are cutting into the 'Standby aka Shared' RAM which in turn hampers RAM 'cache' to the GPU(s) if used for rendering.
RAM is dirt cheap! Match your exact 2x4GB kit installed now. BTW - I 'render' SQL and with 32GB I have to cut the jobs into pieces; so it depends on the size project. Again, 'Free' is the measure to update.