With just a VX-7 you are going to have to do things the old fashion way, they are most likely using the 70cm band for point to point communications (if they have not gone digital), which is 450mhz-470mhz. So plug 450 into your VX and start rolling through the freqs with a 10khz step. It may take awhile but it works. Do it in the restaurants parking lot, while there are customers, so you know when its active. Most have a tone that alerts the workers when a car pulls up. That's the dead give away you have the right freq.
I'd recommend using a 12.5kHz step for the 450-470MHz band. This is the standard step for UHF NFM transmissions. Using a 10kHz step will cause you to be off-frequency 75% of the time. If you're close enough it may not matter and it will just be a weaker, distorted signal. The VX-7 also has a feature called "Channel Counter" which will lock onto a nearby signal even if you don't know the exact frequency, just as long as you can get it close. You can change it from a 5MHz swath to a 100MHz one, but I'd recommend the 5MHz as the smaller the range, the weaker the signal it will detect and drive-thru systems tend to not run a whole lot of power. Read the manual for exactly how you accomplish this. Also, the systems I've encountered in this frequency range often only transmit when a car is at the speaker, or when the employees are talking from headset to headset so that makes it more difficult to find. Others will transmit all the time.
There's other ranges you might want to scan, like 30-35MHz at a 5kHz step, and 150-156MHz although these are usually inputs. Once you find one frequency, you should google it to find the other frequency as they usually use standard pairs. As for the 450-470MHz ones, the frequency pairs are usually 5MHz off from each other. Googling can also help you with CTCSS tones. Doing it on your own, you should use the built in tone decoder on the INPUT frequency to determine the correct one. RTFM on how to do that. You could alternatively transmit every PL tone until you hear yourself on the output frequency if you have another radio, or you can just drive up to the speaker and listen until you hear something come out of it. Some also transmit a quick DTMF tone along with the CTCSS. One I've heard used a DTMF "0" to talk to the speaker and "#" to talk from headset to headset. You can either hit it quick when transmitting, or set up the WIRES feature to automatically do it for you every time you transmit.
If you have a particular fast food place in mind that you want to take over, you can find out their models of headsets by calling them and asking. Say you're with the Radio Enforcement Agency and that you've had complaints about their headsets interfering with local frequencies. Get them to take off their headset and tell you the model number. A Google search will then tell you which frequencies they run on.
Before this stuff was all over the internet, I called a fast food restaurant and got the manufacturer and got them to read the channel/tone code sticker off of the unit above the drive thru cash register. I didn't say I was from anything; they just obeyed without question! Then I called the manufacturer (in this case HME), and told them I was a manager of a fast food restaurant and wanted to know the frequencies of the system so I could monitor my employees with a scanner. They were able to tell me the frequencies in and out based on the single letter codes that were on the sticker of the unit. They wouldn't give me the PL tone though. Anyway, make sure to not only get the make and model of the unit, but any channel code that may be on a sticker on the unit. You can still scan all possible frequencies by that manufacturer. If all else fails, an FCC ID will appear on both the base and the headset itself and you can search this on the FCC's site and pull up the manuals and other documents that would have a list of all the possible frequencies they operate on.
