The simple answer to why a vendor would want to accept bitcoins is that any vendor that can offer flexibility to his clients will get more business.

The second factor is that dollars are going down in value, and bitcoins are going up AND down. At some point they may just go up, but you have to take what you can get now. So vendors that amass bitcoins will generally fare better.

However, there is the important principle that "bad money drives out good". That's why people hoard silver and gold, and spend US dollars. US dollars are bad money, so that's the first thing most consumers will want to get rid of as they decline in value. When inflation sets in, US dollars are going to be flying around while other stable currencies go underground and appreciate. Vendors won't be able to get their hands on them until they are the only things that still represent value.