Do vendors need to be put on seating chart?

Yes your photographer or if you have 2, they need to be included with your count for food as well as where you're going to sit for dinner. It will also say in your contract if they need to be included for food. My fiance and I's photographer outlines what she needs for the day since we have her and her second shooter for 12 hours.

For DJ, they should also be included for the food count but you don't need to include them in the seating chart. They usually eat at their dj booth.

They should have a clearly marked table in the same room as the reception (I personally really dislike it when couples "hide" the vendors in a different room or space when they eat) but it's not necessary to put them on the big printed board with their names if for whatever reason you don't want to have them on it.

I've got my photographers, wedding planner, her assistant etc at a 'vendor table' on the seating chart. It helps me keep track of meals for the caterer and gives them a place to get off their feet and have a bite to eat.