Yes... I just spoke to a friend at VNA who said you can upgrade for cash at check-in at SGN subject to availability. She has no idea how much it will cost (she's a purser, not ground staff) but said she'll "try" and get back with an answer. I'll let you know if I hear back. I suppose whatever rule applies at check-in would also apply at a transfer desk.

It seems strange to me they can't issue the ticket in Taiwan - I suppose they don't have a fare because there is no Y+ on TPE/SGN. I wonder if they could price it as Y+ on the basis that you take a voluntary downgrade for TPE/SGN. Might be worth asking... One bonus is that if they could do it that way, you'd be high up the list for an OP-UP to C on TPE/SGN if Y was full.

I can't think of any "tips"... but

1. by being willing to pay for the upgrade at check-in, they might decide to do it for free if they are doing some OP-UPs anyway. (Doesn't sound very commercial, I know - but they wouldn't want you to think you've been cheated by them!)
   
2. join "golden lotus plus" if you can do it in Taipei. I suppose you'll only have the paper-card. Once upon a time they were doing an offer on CDG where all GLP members (regardless of status) were upgraded for free (subject to availability - and seats generally were). This was before they were selling Y+ as a distinct product. I suppose this won't work if you're putting the miles elsewhere.

Do get back with some feedback on the flight if you go ahead. I used to fly TPE/SGN with VNA often (in C). Also SGN/CDG (C) once. I think they're great, and miss them terribly.

I played chess on the 777 entertainment system. Drew the first game. Then won twice. Slowly increased the difficulty level to maximum. Continued to win. Felt very clever (not played chess since I was 12). An FA came up to me and offered me some coffee/snacks (everyone else was sleeping). She asked "are you winning?". "Yes, I answered proudly." Then modestly added "Perhaps I've been lucky". She corrected: "No. Its adjusted to be very easy. We want to give our Vietnamese C pax a good feeling so they don't lose face. I think that's not a problem for you, is it...". Great put down. When she returned with the coffee, I was reading a magazine.