does the Guangzhou veltra bus hop-on hop-off works? where can I purchase tickets and board on?

Do you mean the Sightseeing Bus? There has been three sightseeing bus lines opened, passing many popular attractions. The three lines are in service between 09:00 and 17:00 all the year round. Buying a ticket at the ticket counter near each stop or from the driver with CNY50, one can enjoy unlimited drop-on & drop-off within 24 hours.

Visitors can transfer to different lines at Beijing Road Pedestrian Street or Dashatou Dock. Free audio guides are offered in eight languages onboard.

I just got off the veltra bus you speak of.
Routes are good people are good price is good that is where it stops. The drivers are smoking more than they are driving.
I got off one route at beijing pedestrian street, didnt think to look, but i got on the bus that was sitting there waiting. Same route i just off no direction from staff, that put me behind 3 hours. I started at 10 am it is now three and only done 2 routes. If you stop and get of anywhere doubtful will be able to do it in a day. No coordination at all with buses.