Is it possible to nest VPN connections?
Is it possible to create an VPN connection while using site-to-site VPN?
Have you ever tried or done this?
What are possible problems, pitfalls, things of the impossible, etc. ?
Also see: Is it possible to use Microsoft's "Secure Connection Rules" (IPSec) with VPN?
You're likely to start having issues related to MTU, but yes, it can work.
Whether it will work depends on what exact combination you're hoping to achieve.
yes, it is. read this very complete explanation on combining tor and vpn nesting.
then read this vpn nesting workflow.