Always try to work with the vendor directly, first.
It's very straightforward for an honest vendor to refund the charge, and the transaction only costs him a few pennies at most.
Chargebacks hurt the merchant unduly
If you initiate a chargeback, the merchant is immediately charged an irreversible fee of about $20 simply as an administrative fee. He'll also have to refund the charge if it's reversed. To an honest merchant who would've happily refunded you, it's unfair and hurtful.
In any case, now that he's out-of-pocket on the administrative fee, his best bet is to fight the chargeback - since he's already paid for the privilege to fight.
Also, a chargeback is a "strike" against the merchant. If his chargeback rate is higher than the norm in his industry, they may raise his fees, or ban him entirely from taking Visa/MC. For a small merchant doing a small volume, a single chargeback can have an impact on his overall chargeback rate.
Chargebacks don't always work.
The "threshold of proof" for a chargeback varies by patterns of fraud and the merchant's ability to recover. If you bought something readily fungible to cash - like a gift card, casino chips, concert tickets etc., forget it. Likewise if you already extracted the value (last month's Netflix bill).
Chargebacks don't quite settle the matter
Credit card chargeback only withdraws a payment method. Your bill is still due and payable. The merchant is within his rights to "dun" you for payment and send you to collections or court. Most merchants don't bother, because they know it'll be a fight, an unpleasant distraction and bad for business. But they'd be within their rights.
Working with the merchant to settle the matter is a final resolution.