What is a foreign voucher that dr.wei said in ch.11?

Interesting question. They're called Foreign Exchange Certificates and they were used in the past in China as an alternative form of currency, rather than letting foreigners have direct access to yuan. You can read more about them here: http://www.borrowedculture.com/2010/11/26/foreign-exchange-certificates/

I'm not sure why Wei Wei would have had to use them though.

Wei Wei was making fun of their huge age gap bc young ppl (Dings age) wouldn't know what a foreign voucher is.