If the Asus does not have VPN capability and the weak Wi-Fi from the Virgin is the only reason you bought the Asus and are OK with all else from the Virgin, then you could turn the Asus into an Wireless Access Point (through configuration) and not have to touch the Virgin (poor pun). IMO, that's the easiest thing to do.
If the Asus does provide VPN capability and you have reason to access your LAN remotely then that should be in your plans, sooner than later. That may entail placing the Virgin into bridge mode (if possible) or jumping through other hoops so that there's no double NAT (both routers don't need to be routing).... a bridge to be crossed at that time
If you decide to go the first route basically you log into the Asus, turn off DHCP (disabling it's router function) and assign a static IP to its LAN in the same subnet as your current LAN (the IP subnet as currently handed out by the Virgin router) and with an Ethernet cable connect a Asus LAN port to a LAN port on the Virgin....now the Asus is NOT routing but is only a Wireless Access Point (the Virgin is still performing the routing / assigning IP addresses). For the static IP you choose to assign to the Asus' LAN insure you use an IP that is outside of the Virgin's DHCP pool. In other word, if the Virgin's router hands out IP's from XXX.YYY.ZZZ.050 to .099, use an IP for the Asus' LAN in the range from XXX.YYY.ZZZ.200 to .254 .
