So you use Tor and tell me you use BobVPN. So now that is great because I can go to BobVPN and see who all is using Tor. Okay there are twenty people using BobVPN and connecting to Tor, you are probably one of them. Now if I can watch traffic over BobVPN I can quickly pinpoint your traffic with fingerprinting attacks, since I already see the posts you make here and the size of the posts. Only one of the twenty people using BobVPN to connect to Tor is likely to have traffic pattern correlating with the posts I can see you making on the forum here. Also many VPN providers keep logs of who is connected to the VPN when, even if they don't keep traffic logs. So now I can use these logs and see who of the twenty are connected to the VPN when I see traffic from you on this site. Over a small period of time I can use intersection attack to deduce who you are. You could say well my VPN keeps no logs blah blah, and I say fine that is great but you still have essentially changed your security model from Tor to your VPN provider, and that makes life easier for me if I am trying to trace you.