Plain Prinia - Van Long

I think it's a Plain Prinia

Thanks. I had tentatively put this into the Acrocephalus genus, so I'm somewhat encouraged that I wasn't a million miles away

yep, it is definitely a warblery-looking bird so you were very close. If you google photos of plain prinias most don't look quite the same (they tend to look much cleaner), but the ones at Van Long all seem to have the buffy face like yours.