I'm not Catalan, but IMHO "vaig comprar" means "I bought", and "jo comprava" can mean either:
* "I was buying"
- Què feies que no agafaves el mòbil? [You wouldn't pick up the phone, what were you doing?]
- Comprava. [I was shopping.]
* "I used to buy"
- De petit, sovint em comprava llibres de novel.les negres. [When I was a kid, I often used to buy mistery novels.]
Sorry for the bad examples.