Quotevastly different
-- well -- not compatible
try https://www.daycounts.com/vm-migrator
or
for users to work they first have to be present as joomla users in J3 .. so I guess you need to migrate them thru any of the Joomla migration techniques
then you need to get them from VM1 to VM3
I can't remember what the old VM1 tables were like.. if similar to VM1.1. then using the std. VM1.1. to VM 3 migration for users might work
just try the vm1 tables in the J3/VM3 db (the table names are different so wont clash) .. then try the VM admin tools
if it wont work another possibility is to get them into Joomla and put up with the fact they will need to re-enter there address details etc (assuming no orders are required)
or contact