By vignetting, you mean light from the flash wasn't covering the subject sufficiently? Can't you zoom the flash head back, use the wide-panel, or move the camera position back and increase focal length?
If none of that works, then a diffuser cap is certainly an option and indoors (with a white ceiling - point it straight up) results can be good and easy, certainly better than you're getting. Press photographers tend to use them pretty much 100% of the time. Stofen is the original brand name, though they are lots of clones about that work just as well for a few quid on Amazon/ebay. An alternative is to use the little pull-out hilite panel built in to the flash head - it works in a very similar way to a Stofen. If your gun doesn't have that, it's a simple DIY job with a small piece of white card and a rubber band, known as a bounce card (google it).
This is a big subject though, and there are loads of other accessories available. Don't believe all you read - it's a area rich in snake oil. Accessories I rate include the Lumquest QuikBounce, Rogue FlashBender, and Neil Van N's DIY Black Foamie Thing. With the last two, you need to know what you're doing for best results.