Sounds bogus, or at least, misconfigured to "block viruses"
As the PGP folks said in so many words, their software's output is just a binary file. The VPN is just another network layer on top of TCP/IP, and should not care what files are being transported. If there truly is an issue, it implies the VPN is misconfigured to filter out binary files, probably in the mistaken idea that doing so will keep viruses from transporting across the VPN. The experiment to try is to see if one can send the PGP file "as is", and again after MIME encoding. (MIME = Multipurpose Internet Mail Extensions) All email is transmitted as US-ASCII plain text. Period. The way binary attachments to email are transmitted is that they are first encoded to an output that is all US-ASCII text. The most common modern encoding scheme for this is MIME. Another encoding scheme not used much anymore is UUENCODE ("Unix-to-Unix encoding").
In MIME encoding, the input data undergoes a mathematical transformation from base-2 to base-64. In this conversion, the output results in 4 bytes of output data for every 3 bytes of input data. But the good part is that every single one of those output bytes is a simple plain US-ASCII text byte. Hence, the MIME encoded file can be sent as part of an email and everything between the sending client and the receiving client sees nothing but US-ASCII text. The receiving client reverses the encoding to produce the original attachment file.
If the MIME encoded file transports across this screwy VPN but the binary PGP does not, then they are blocking the transmission of binary files probably thinking this impedes the spread of viruses across the VPN. (Bad assumption.)