Many times I've asked the photographer for the host at the party to take pics and email them to me. Once in a while they will, but not always, so I can't depend on that.
It's really not a good idea to do this unless you want to alienate your client and the photographer. A professional photographer is working for his (and your) client, not for you. He/She is there to take pics of the guests celebrating their event and having a good time, not the food. Of every 10 shots, there might be one usable one that he will get paid for so he needs to maximize the number of available photos that are sellable to his client. It's unfair to your client and the photographer he/she hired to expect him to spend time photographing your food for free. Additionally, the photos of your food probably won't be that good from a marketing perspective.
If you want to build a portfolio to show prospective clients you should hire a photographer yourself-preferably one who is familiar with photographing food. Lighting, camera angle and POV will have a huge impact on the quality of the shot and whether the food actually looks appetizing. Steer clear of the massive trays showing piles cold cuts, crudite, cubed cheese and fruit lying on a flat table. Nothing is really more boring visually and it does little to distinguish you and the services you provide from anyone else.
Get specific by showing small plates of easily recognizable food that represents what the client might expect when booking a party with you. Show happy people eating your food. Show foods that relate to each other stylistically, like a couple satay skewers and a lettuce wrap, or a couple of pretty shrimp with some kind of salsa. Don't show the low-priced options-you'll get plenty of customers who want cheese trays and crudite. If you're going to spend the money on a photographer, show off your best work make it interesting visually.