is possible to remove or disable by default from /opt/vyatta/share/perl5/Vyatta/SrcNatRule.pm
if (!($outside_addr_mask == $src_addr_mask)) {
return ("\nsource address should be a subnet with the same network prefix as translation address" .
"\nwhen translation address is defined with a prefix for static network mapping "
, undef);
}
You already can do this by entering IP range as translation address.
When you use network address as translation address, then VyOS trying to configure NETMAP rule (not SNAT).