Firstly, I would strongly disrecommend against building a cruise control unit for your car, without having experience and completely knowing what you're doing; overriding the throttle with a self-made construction may very likely cause your 'unaware' engine system to get unstable. Because it is not familiar with your construction, it will most likely try to compensate for it, go to limp mode, and throw all kinds of errors. But if you're determined to go through, here's my advice.
Learn about PID's and plausibility checks
Cruise control mechanisms make use of a PID: a Proportional Integrating Differentiating controller. This is a software closed loop system that applies more or less throttle depending on the vehicle's speed. It tries to keep the speed constant. The problem is that this system has to be very robust, i shouldn't be so that one glitch in the vehicle speed sensor triggers a full throttle, because your loop thinks it has to compensate that glitch. You would get a very unstable system. So, you have to do a lot of filtering and plausibility checks in your software. I recommend to google and learn about PID's, the Ziegler-Nichols method is a good start to get familiar with the PID. Try to find a program where you can test with a PID. There are some Excel's available online that can help you. You'll see that it's very easy to get a unprotected system unstable.
You can think for yourself that it would be silly to fully depress the accelerator if you see that the speedometer suddenly reports 0 mph.(because it's broken)
Software doesn't think like that. That's why there are loads of plausibilty checks needed before the software gets to depress or release the accelerator.
Familiarise yourself with the term plausibilty checks and system robustness and you'll understand what i'm talking about.
Implement failsafe mechanisms in your design
In reality there are numerous fail safe mechanisms used; obviously the cruise control disables when either the clutch or brake pedal is used. Mostly shifting gear also disables the system. It's wise to put a switch on your dashboard that cuts off all your self-made wiring so that the engine system falls back to the state it would be entirely without your construction.
Engine management is made extremely robust, so chances are your engine management picks up the tasks you did override, and resumes without further problems. It's still dangerous though.
Get a full understanding of the brake booster
Regarding your question about the brake booster, at full throttle manifold vacuum falls to (near) 0. There's a vent on your booster to prevent air goind from the manifold to the booster. A charged booster allows you to brake 2-3 times after vacuum has been lost. If you ever get in a situation where your engine fails at high speeds and you lose vacuum, brake firmly(after checking your surroundings) and try to let go of the brake pedal as little as safely possible. Each depression of the brake further depletes the booster. Keeping the pedal pressed and still doesn't deplete it further.
The more closed/unpressed your accelerator is the higher the vacuuum, and the higher the rpm is the higher the vacuum. And vice versa. You can see it as a function Vacuum: V = - throttle% * rpm. (it's more complicated in reality)
Check your manifold pressure while revving with an OBD scanner to get an idea.
Test with baby steps
If you're determined to try out your construction, i'd recommend to first extensively test it in first gear and not on public roads. In 1st gear, the engine won't be able to get you any faster than ~50km/h.