Hello
I also had the white screen on my development/test site. Since it was a test site, i take the risk to go for some risky stuff:
1- note the invoice number
2- go to vmfiles/invoices and delete the invoice if it exists
3- in phpadmin, go into Jxxx_virtuemart_invoices look for the invoice number and delete the corresponding invoice entry
4- try to print invoice
This worked for me. No idea of what the actual problem is.
Gerard