I wanted to do the same thing a few days ago. I'm not sure if there is any better way, but I found the following worked for me:
1. edit shopcustadminlogin.asp (about line #32) ....
If GetSess("Customerid")<>"" and Getsess("Lastname") <>"" then
If getsess("Customerlogincid")="" then
msg=getlang("langLoginSuccessful") & " " & Getsess("firstname") & " " & Getsess("lastname") 'RW-29.08.2005 ~ added welcome personalisation
responseredirect "shopcustadmin.asp?msg=" & server.urlencode(msg)
See where the msg gets formatted. I added to this with the customers firstname and lastname. There are many other session variables that you can use (these are available when the customer logs in). A list can also be seen with diag_sessionlist.asp.
2. In addition to this, you need to edit the languages table. Find the keyword "langloginsuccessful" and you edit the caption to what ever you like to preface the customers names above. i.e "Login successful, Welcome back, ".
As the languages are loaded at VP-ASP's initialisation, I found it necessary to re-start your pc for them to reload.
I hope this helps..