The VyOS command language accepts a community string for SNMP. SNMPD, though, has different community strings for ipv4 and ipv6.
VyOS 1.1.8 correctly set both 'rocommunity' and 'rocommunity6' to the same value, but the re-written, python command parser for it doesn't set 'rocommunity6', meaning that snmp over ipv6 isn't possible.
The fix is simple - a new line in the parser template will resolve it. I'll try to get a pull request submitted to do exactly that.