You should install and configure VTun package and read documentation. VTun package contains several config file samples. UDP doesn't not provide reliable transport, and some frames could be lost on unreliable links. VTun does not have any retransmition or recovery algorithm for the lost UDP frames, because retransmition will be handled by Upper layers. You should use LZO compression with UDP protocol.
Explanation is the same as with above case.
Use TCP for PPP tunnels or use TUN instead of PPP, since tun device is completely the same with ppp device for the kernel. Actually there is no straight way to determine when pppd has established connection with the other end. VTun execute those route commands right after the launch of pppd and if PPP link is not established, route commands will fail. Since version 2.0b3, VTun delays 10 seconds before issuing the route commands, which should fix almost all cases. I'll think about more elegant way to fix this problem.