Gyms in Vietnam are very variable in terms of their size and quality, ranging from top notch gyms with facilities as impressive as any you could find in Western society, to small local ones with varying degrees of quality of equipment, but mostly of a usable and functional standard. English is mostly spoken to at least a reasonable degree in touristy areas, but may be basic elsewhere.
Average price was 50-100k dong, with one particularly top end gym claiming to charge 400k for a one-off casual visit. Small towns or less touristy cities commonly charge under 50k though – my cheapest gym was just 10k!
Currency approximation : US$1 = 23k dong, €1 = 28k dong, £1 = 32k dong, AU$1 = 17.5k dong
at the time of writing.