Hi Paul,
It's not possible to set QoS policy specifically for a VPN tunnel, but if traffic is already marked or classified by the SRP then this marking will be copied to the IPSec header and then queued in accordance with the QoS policy setting for the WAN interface that the IPSec tunnel uses.
In other words, if you have RTP traffic via the VPN, this will be prioritised over general internet traffic (not in the VPN) when leaving the SRP.
Regards,
Andy