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.