I don't think there is a good answer to this - given that MasterCard and Visa are blocked, i.e. that one can't pay for the VPN without external help. Free VPN providers are going to be enumerated and blocked. I think the most practical solution would be to get Tor (email to gettor@torproject.org to get a personalized non-blocked download link, or get Orbot from F-Droid on Android while it still works), and use it with a non-public obfuscated bridge.

If you still have a foreign SIM card with sufficient balance (or can order one through Vietnam while paying with a Mir card), try to use it, this is also an effective (but expensive) censorship circumvention device even in mainland China.

P.S. I have already relocated to the Philippines, so the advice above does not apply to me.