Hmm, actually Hero Duet VX doesn't have a seat height adjustment capability. AFAIK on all the scooters, only Mahindra Rodeo line has this feature. Mainly it doesn't matter for average heighted ladies. But for someone of about 5Ft height, the default seat height of 70cm can be a bit too high. And my wife belongs to this group.
Thankfully many accessory shops help you with the seat height. After asking around we found couple of shops nearby and visited one of them for the change. It's simple, they shave bit of foam off the saddle to get it low. It cost us INR 150 bucks.
But actually there should be an option for seat height adjustment for everybody. I don't know why bike manufacturers (except Mahindra on Rodeo) don't provide this.
To look at it other way, your plain jane bicycle provides seat height adjust!!! J