accel-pppd supports sending accounting interim updates to RADIUS as indicated in the following document: accel-pppd
It indicates the following can be applied in the [radius] section:
acct-interim-interval=n By default is not defined.
Specifies the interval in seconds to send accounting information (may be overridden by radius Acct-Interim-Interval attribute)
This configuration setting is not supported in the VyOS CLI from what I can see, nor is the change persistent upon reboot when modifying the /run/accel-pppd/l2tp.conf file.
It would be great to introduce this configuration option so accounting information can be sent to RADIUS.