Thank you Benjamin for your reply. I am pretty sure v.vol.idw was available at some point even after the introduction of the new vector format (v rather than s points) - if only I could remember for which version of GRASS. Moreover, it is mentioned in Neteler and Mitasova's GRASS manual (including the recent 2008 version). That is why I was surprised the command does not seem to exist in recent versions of the software.
I am afraid I do not have the skills to convert it. Is there anyone out there that could help?
All best,
Stefania