Ok it appears Latvia and CZ are out:

"As of October 2017, fifteen European countries legally recognise and perform same-sex marriage: Belgium, Denmark, Finland, France, Germany, Iceland, Ireland, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden and the United Kingdom. In Austria, marriages will be legal from 1 January 2019".

Out of those I don't know which ones would readily grant a Visa to a Vietnamese citizen. Denmark is mooted online as the easiest to get married in but the ease of obtaining the Visa is not stated and the list of documents required for a marriage is quite onerous