No, it is not required to use DVTI on either of the easy vpn sites, whether it is server or client.
You can still use the traditional easy vpn configuration, or you can use the DVTI configuration.
The reason why it's not working with DVTI is probably there is a numbe of bugs with DVTI, and sometimes you would need to upgrade to certain code to resolve the issue. But if you don't require DVTI functionality, and the traditional easy vpn works just fine, I would suggest that you keep using that.
Hope that answers your question.
