Sounds like you are talking about a Vespa PX - according to the manual you should get 260Km on a tank. I think this is if you are a very small Italian and travel at a constant 60km/hr. Specs for newer Vespas like the GTS 250 have to conform to EU measurement standards and therefore are more realalistic. Your 150 km is about what I get for mixed riding - I get 130 Km full throttle on motorways.
Agree with the other answer (tire pressure, weight, etc). In addition you can try a windscreen to reduce your drag (or drive around in a race position). Tyre pressure reduces rolling resistance but aerodynamic drag is your biggest enemy. Also avoid puffy jackets (you should be wearing a motorcycle jacket for protection anyway).
As an example my wife and I road for 100km on similar Vespa's, I had a screen and she didn't, I used 1 litre less. The screen also makes you go faster :)