shifting frm webmin 1.150 to webmin/virtualm3.50

Wow, that's an old Webmin! ;-)

Depends on how much you want/need to bring over. If it's pretty simple websites, without mail and other services that also need to come over, you could just copy the VirtualHost sections over to the new box, and use the Import feature in Virtualmin--this allows you to pull in existing VirtualHost entries and use those to build new Virtualmin accounts. You'd need to fill in all of the missing stuff, and you'd probably have to manually copy the data into the right places once created. You might also want to modify the paths in those VirtualHost sections to point to /home/domain, if they don't already, so that your installation matches a "default" Virtualmin installation (and so that suexec, fcgid, etc. works without much pain). Frankly, this isn't a lot less pain that creating them from scratch.

Another option, if you do have a relatively complex setup, with mailboxes, users for each of the virtual hosts, etc. would be to upgrade Webmin (you should do that anyway...I'm sure a Webmin that old has some bugs and security issues that have been resolved in newer versions), install Virtualmin, and use the Import feature on the original box. Importing is generally non-destructive, and so it would just be bringing the existing virtual hosts under control of Virtualmin, making them possible to backup. Those files could then be restored on the new server. If the system is reasonably similar to a default Virtualmin installation, this will work flawlessly...but everything that is different will throw it off some and require some hand-massaging during restoration and afterword, as well, probably.

Because there are so many possibilities when setting up virtual servers, with mail service in particular, it's practically impossible (literally...there is no practical way to accommodate every possible configuration) to say "here's how you do it, and it will work fine". Webmin, without Virtualmin, imposes almost no standards on how things are configured. This is a both a major strength and a weakness of Webmin, and in this case it means you're probably going to have to do some work to bring these virtual hosts under control of Virtualmin.

You may also want to try out the batch modes of Virtualmin. If you just need to create a list of Virtual Servers, just feed it to the command line create-domain.pl script with a bash script. Or use the batch creation mode of the UI to feed in a colon-delimited file with the server details--the format is documented in the help on the batch creation page, and in the Documentation section of the website.