Try the Eunos Industrail Park. Just next to the Singapore Post Centre.
Depending on the vespa you want.
I got one at $400/- Sold it off at $800 after 6 months of riding it.
So you surely wouldnt want to buy from me
It will depend on its condition lah.
Those pretty done up antique vespa of course cost more. Usually starting from $1000.