Does anyone know how the traffic behaves if you configure a VPN natively through SSTP (WIN or MAC)?

Because most applications use the TCP, but Skype uses UDP.

I try to formulate more specific questions:

1. Is it true that the VPN works over TCP or via UDP, but not both at the same time?
2. If I set up VPN over TCP, is not there some kind of IP leakage in the case of using Skype (UDP)?