My goal is to create a secure tunnel for credit card transactions.

POS--->mikrotik---->vpnclient---->public wifi network--->Routeros on a VM with tunnel server---ether2 (payment processor).

Everything works for this setup until I throw in the fact that their will be multiple POS machines. I would like 1 set of credentials that can be used amongst the many Mikrotiks that will be sitting at the POS devices. PPTP seems fine for 1 connection but when I create a secret with "local address" and "remote address" specified it tries to use the same remote address on any device that uses those credentials to connect.

Is there a different tunneling method I should be trying out?

If you don't have too many POS terminals, just add a "secret" for each of them with unique address allocations. Each POS terminal nill need to use unique credentials to log in.