That would be the answer - except that he is specifically not allowed to get married or enter a civil partnership in the UK on a Visitor's Visa, nor does Vietnam recognise same sex marriage, so that's out too.
A few years ago it was trumpeted that Vietnam was going to lead the way in Asia with same-sex marriage. The reality was different and turned out to be that you can now go ahead and have a nice little same-sex wedding ceremony in Vietnam if you feel like it (nobody will stop you) but legally it means nothing, therefore the UK will not recognise it, so bang goes that Visa option.
I really feel that after 7 or 8 years to get him here we are not going to be split up by red tape, so if I can't keep him here legally (and he desperately wants to stay and wants nothing in the way of state benefits) then I may have to just bite the bullet, sell up, and move elsewhere.
As one gets older the limited time left really concentrates the mind and one realises there is very little time to piss about before one's health inevitably dictates that all options have run out