If you're on different non-overlapping channels, there will be little interference. The three non-overlapping channels are 1, 6, and 11. However, Ch 3 overlaps onto Ch 1. Therefore, I suggest you move your Ad-Hoc network to Ch 6.
The mostly likely culprit is that your route changes when connected to whatever wireless access point appears and is probably the remote gateway for the VPN. Assuming you're running some Windoze mutation, from an MSDOS window, run: route print to dump routeing table. Try to figure out where you packets are really going. If you're not sure, try running: tracert ip_address where IP_address is some address on the internet. It might show the route packets take to get through the VPN, to the internet, etc.
Also, I once threw together a wireless repeater crafted from a pair of DWL-520 PCI cards in one machine. The antennas were about 2" apart. The radios were on Ch 1 and Ch 11 for isolation. There was no obvious RF interference problems with this arrangement, although I didn't do much testing beyone moving the antennas around and using the site survey tool to check for S/N deterioration.