Mia noi would usually be a mistress with financial support and I guess some older guys with young girls as 'giks' may support them but to me this isn't really gik-ship and more sponsorship.
But yeah many people use it in different terms.
So the burning question is...are there plenty of Vietnamese girls under 30 who are happy to have a no strings attached 'gik' style relationship with a foreigner without receiving support / sponsorship / marriage not being on the cards?