For some readers this is never an option but depending on your beliefs and if it ok with your god, why not participate in your temple visit instead of just being an observer ??
You see, since Vietnamese religion is such a mixed bag of different practices, they are quite ok for a foreigner to pray at their temple.
Often they will sell imitation money (ghost money) outside of the temple, so you could try burning some money yourself in the outside fireplace (someone will show you where), lighting them with a lamp or candle, then imagine sending it to your passed on loved ones.
If you do want to try this (and it is really ok with your god), the person selling the paper money will also try and sell you some incense/joss sticks, flowers and a candle. So, just put the flowers in a vase inside the temple, light the joss sticks and candle by a lamp or light it from another candle.
Say a prayer holding the lit candle and joss sticks up to your chest, then place the items where other people had put there’s. Sometimes they are placed outside in front of statues, an alter, trees etc.
While you are there . . . . .
It also cost a lot of money to maintain these ancient structures, so if you feel inclined it would be a nice gesture to donate some money into the donation box. It doesn’t need to be much, say a $1 and all those $1 notes can all add up to something at the end of the day.