Thomas
I can not think of any reason why it would matter whether the VLAN were tagged or untagged. What the site to site VPN needs is IP connectivity. As long as the ASA on one side has appropriate IP connectivity to the other ASA then the VPN should work.
You could assign them a separate VLAN and they might feel good if you did. But I do not believe that it is necessary or adds anything significant to protecting the traffic. The IPSec VPN provides protection for the traffic no matter what VLAN it is in.
HTH
Rick
HTH
Rick