The Vietnamese pronunciation of the character is in the nặng tone, which means it ends in a glottal stop. It corresponds to the entering tone, and exactly is what you would expect the character should sound in Vietnamese, where characters that begin with a fricative and affricate in Middle Chinese would instead begin with a t. In my opinion that’s reflects the older pronunciation, because I think the sound change order is /t/ > /ts/ > /s/.
In Taigi, the final glottal stop is lost, and many people are pronouncing the word as siā, but plenty of older speakers, especially down south still would say tsiā. The dictionary would say tsiā is the colloquial reading (usually the older reading), and typically for last names we should use the colloquial reading.
Back to Mandarin, there is absolutely no reason for it to be spelled as Hsinh.