My 150cc Genuine Stella 2 stroke (made by LML in India) has a very low top speed of about 50 mph. And it cannot be ridden at full throttle for very long at the time without blowing the engine. A cruising speed of 40-45 is more like it. A 50cc Japanese scooter can do that. And my Zuma 125 and Vino 125 blow it away, hitting close to 60 top speed, and they can be ridden that way all day. The one place the Stella shines in in the mountains, where it's manual transmission allows you to keep the engine rpms up while climbing slow. It doesn't bog down, and beat up the engine. I don't know if a 150cc Vespa is any faster, but they are much higher quality, so you can run them at full throttle a lot longer.
I won't spend more on a bike than I think it's worth, but if it's a good deal, I don't seem to have a problem buying bikes I don't need.
2002 Vulcan 750, 2013 Royal Enfield B5
2001 XT225, 2009 Genuine Stella
1980 Puch moped