Right now SSTP, PPPoE, and other protocols backed by accel-ppp only allow RADIUS server addresses to be IPv4:
vyos@vyos# set vpn sstp authentication radius server Possible completions: > <x.x.x.x> RADIUS server IPv4 address >
That's a serious limitation for companies that use IPv6 or want to add flexibility and redundancy by using domain names instead. We need to test if accel-ppp can work with them and relax the restriction. Official docs only mention IPv4 so far, which is where the restriction comes from.