My brakes started vibrating occasionally when slowing down on highways at high speeds. I have been realizing that the brakes feel low. I went to a shop for an oil change and the mechanic peeked in between the aluminum wheels & told me that I definitely do NOT need to cut my rotors but I do need to change my brake pads. Why would I be vibrating when braking if there is no need to cut the rotors? and is it possible that I only need to cut the front rotors and not the rear?
Brakes that vibrate when engaged, worse at higher speeds indicates the rotors are warped. This can happen for many reasons, hot brakes and drive through a cold water puddle for example. If the steering wheel shakes when you brake, it is the front brake rotors with the issue. If the steering wheel does not shake it is probably the rear rotors. The mechanic that looked through the wheels and said you didn't need the rotors machined was probably saying there was no unusual wear and/or the brake pads have not scarred the rotors. The only way to determine which rotor (or rotors) is to blame is to use a dial indicator to measure the rotor face while rotating the wheel. You can have them machined, or you can replace them. Some rotors are very inexpensive and cost about what it takes to have them machined.
As for the low brake pedal, that is another issue. You may have a rusted/stuck caliper slide or, depending on the style of emergency brake, the rear calipers could have an internal emergency brake related issue. More information would be needed to go forward with that answer.
I am assuming there has not been any recent work on the brakes. A brake pad not fully inserted into the holder could also get "caught" and cause the low pedal issue.
Cutting rotors is required when the surface is not uniform - has grooves from old pads. If your surface is smooth, you don't need to cut.
Yes it can be the case that you need to cut only front or rear rotors.
Vibration will go away on it's own, but you need to adjust your braking habits - if you are braking hard, try not to stop and keep holding your brakes - better roll a couple feet very slowly to not let the pads glue themselves to hot rotors.
start from the outside in: first check your lug nuts to make sure they are all tight. loose lug nuts can cause the rim to wabble. jack up the car, Remove the lug nuts, and remove the tire and check the brake pads for wear. You also want to feel your rotors from behind. cop a feel at the back of the rotor disk slide your finger straight up and down to feel for any mounds. The rear of the rotor surface should not have any mounds on it/ on either side. If you feel any mounds then it is time to replace that rotor. Rotor dust can build into mounds when pads are constantly changed for years but the rotors are not changed un seen dust will build under the pads and slowly mound up and begin to create grooves into your pads. This could happen on just one side or both but usually happens on the inside under-carriage where the most dirt attacks. Chek out this video for more Information.