I have found a better solution for this problem, using squid instead of firewall rule. I did get success using firewall rules but I have to include every google and youtube ips I found in arin, not an ideal solution for me. Now I have set squid to use gateway of the vpn and firefox add on foxyproxy only to use the proxy for youtube and it works without a hitch :)
Here is how i do it if anyone needs to know
add this rule to floating rules
interface : vpn interface
direction : out
protocol : tcp/udp
source : any
destination : any
destination port : squid port
gateway : vpn gateway
in proxy server general setting
interface : lan and loopback
custom option : tcp_outgoing_address 127.0.0.1;
and you are good to go, set your browser to use the proxy and every traffic to the proxy will go through vpn gateway
I didnt make this soution but found it here in this forum, it is the same setting with proxy for multi wan.