It seems we already have mfa T3834 but it never was documented
https://github.com/vyos/vyos-1x/pull/1008
vyos@r4# set interfaces openvpn vtun0 server mfa totp
Possible completions:
challenge Expect password as result of a challenge response protocol
(default: enable)
digits Number of digits to use for totp hash (default: 6)
drift Time drift in seconds (default: 0)
slop Maximum allowed clock slop in seconds (default: 180)
step Step value for totp in seconds (default: 30)