As far as I was aware, VPN client support is built into win98.

I found this http://www.ncl.ac.uk/ucs/vpn/vpn98.html you might be able to adapt it to your needs.

I'm not really sure what you're after from a vpn but if the only thing is file transfer, you might wish to consider FTP as an alternative, try searching download.cnet.com for software.

If you are worried about the security aspect you could try ssh along with the related sftp software from http://lexa.mckenna.edu/sshwindows/ this encrypts all the traffic including passwords.