Vibrating panties can be hard to buy - mainly because all women measure slightly differently - the clitoral stimulator can be too far forward, too far back, too direct or too oblique. Is one of the main problems women seem to have when buying a vaginal stimulator with attachments designed for clitoral stimulation.
I would advise going for a wireless internal vibrator something kinda like
Wireless Egg
They tend to be much more effective (for me anyways). Just make sure when they're not in use you clean them and remove the batteries or they do tend to go dead quickly (the batteries not the toys)
On a side note if you're shopping for vibrators there's a company around atm Lelo - swedish made vibrators that come with a year long service guarantee, they're probably a bit pricey for a FwB unless you guys think your arrangement will last long term but they're absolutely amazing!
Lelo: Lust Objectified
I hope that doesn't count as company plugging? I don't have any shares in it or anything.
Happy hunting!