I'm sorry for the experience you had. The meter should be in thousand VND, so 100 means 100,000 VND which is a bit less than $5 (1 USD = 23,000 VND). But did the driver make you pay in USD or VND?
What I recall is that the meter didn't cut off the last three zeros but rather just two. Maybe it was a faulty meter (or maybe it was me). I was just off the plane, it was middle of the night and I was exhausted.
I should note that it was me who suggested the wrong amount, and he just accepted. I don't think he set out to swindle me.
Overall I found the country to be mostly free of people trying to take advantage of foreigners - Unlike Thailand and (especially) Cambodia. I also felt very safe there.