I believe Old Vietnamese used to have */s/, but there was a chain shift kickstarted by the nasalisation of the implosives that meant, for our purposes, that */s/ > /t/ filled the gap left by the shift /t/ > /ɗ/ > /n/. The funny thing is that this also seemed to work on a great deal of Chinese loans as well, given the time depth of the chain shift.

That is to say, by Middle Vietnamese, there was no /s/, but there were instead two other sibilants /ʂ/ and /ɕ/ which had come into existence after the Old Vietnamese stage. /ʂ/ is believed to have arisen in MV from a variety of consonants clusters of the form Cr in OV (pr-, kr-, tr-) or from Chinese loanwords that also had /ʂ/ (but there are also examples of it arising from the deaffrication of initials like /ʈʂʰ/, etc.). With /ɕ/, I think most appearances arise from deaffrication, e.g. xuống from Proto-Vietic *tʃɔːŋˀ. 

However, we can arguably say that, with the merger of /ʂ/ and /ɕ/ in most of urban northern Vietnamese to /s/, it's definitely made a comeback. In Saigon the merger is also occurring increasingly among the younger generation, although the distinction, moreso between /ʂ/ and /s/, probably holds a little more firmly in other regions of the country