How credit card friendly do the vendors tend to be? Should I have cash on hand, or would I be safe with plastic?

Also, a small tip:  Be sure to call your credit card company to let them know you're traveling, even if you're somewhat local.  I understand that the purchases can ring up as from the vendor's home state, or to your credit card company it can look like you're all over the US in one day.  Do don't want to have your purchase denied and need to call them from within the vendor hall.

Most of the vendors take CCs; some may not take American Express though.  Keep some cash on hand in case of connection issues or the rare vendor who doesn’t take cards.

Thanks guys!

Fun story, last year I used the bank machine in the dealer hall when the guys that do Burning Wheel let me know they don't take credit cards.  My bank apparently flipped out and straight up _cancelled_ my card due to fraud concerns.  That was a fun trip with zero cash.

So...yeah, be 100% certain to let all your financial institution know that you're travelling.

There are a few vendors that only take credit cards. Caught me by surprise last year when Kosmos wouldn't take my cash only credit cards. It might be an Essen thing I hear that robbery of cash boxes at Essen is a real concern so that might carry over for some.

I generally use a credit card in the vendor hall, and have not yet run into a situation where a vendor didn't accept plastic, but I've gotten a deal more than once for using cash instead. Some dealers are happy to reward you for not having to pay a fee to their credit card processor.

Smart to carry a day's worth of cash anyway, for worst-case scenarios. 

Two more thoughts re: cards getting canceled when you're out of town or making a lot of out of unusual purchases.

• My bank informed me once that the chances of a card getting suspended for fraud when making purchases out of the ordinary are much lower when you run the card as debit rather than credit, because it requires you to enter your PIN number. This adds another layer of verification that the charge was initiated by the actual card holder.
  
• Keep your 800 numbers for your bank somewhere safe and separate from your wallet, so that you can call and put a hold on the cards if they do get lost or stolen. If you informed the bank you're traveling to a convention and that they should allow those charges to happen, you won't have that normal layer of automatic fraud protection.
  

If debit card is your only option, you may want to check with the hotel about their policies, so you know what to expect.

Also note that this doesn't have to be the card you paid for the room with, just the card you have on file at check-in for insurance and incidental charges.

I have yet to come across a booth that didn't take CC but I always pay in cash. I remember one year, opening day had a lot of line congestion because of card declines. So if you do go with card, I would re-emphasize what was previously said and call your bank prior. 

I have also never encountered a vendor who doesn't take at least Visa and Mastercard. However, I also agree that sometimes you can check out a lot faster or even get a deal if you pay with cash. I also carry cash for incidentals and small things so I'm not running my card a dozen times a day. Maybe it's old-fashioned, but I also take my checkbook (yes, some people still use paper checks). Not all vendors take checks but I bring it just in case I see a big-ticket thing that I may be able to get a cash discount on without actually having to carry tons of cash all weekend.

A bit of insight from a guy who sells things at festivals. Most all vendors will take credit cards thanks to the Square Reader. Square will send you a free CC reader and makes their money off the card swipe. Its a great product plus a fair rate; 2.75% per swipe, or if entered manually 3.5%+15 cents. However I'm sure that most venders would rather not pay that fee if given the chance. So its always worth asking if they can give you a small discount if you pay cash. Can't promise they will, but always worth asking :)

FWIW - the vendor that didn't take credit cards was the Burning Wheel RPG folks.

The Auction/Consignment Store specifically gives a discount for paying in cash in the form of whatever percentage (3%?) credit card fees they have factored into their normal prices.  I personally always buy more things there than anywhere else, hence I always pay in cash lest I end up ... well O.K. I have probably never saved more than like $10 this way, but it still helps!