Make some phone calls. Some auto parts houses rent stuff for free. If you have to buy one, a vacuum gauge is the least expensive gauge you can buy. A new one at Autozone is $16. When we compare your vacuum readings with a car in good operating condition, we will know whether or not to keep looking for vacuum leaks. If your reading at idle is normal, we know to look elsewhere. If it is lower than it should be, we can use the gauge to help you locate any external leaks. If you have two vacuum lines leaking, it might be very difficult to locate them otherwise.
If the idle change/oil cap symptom persists, the problem is a vacuum leak somewhere. If it is not external, like a hose, or PCV, then it is internal like a bad gasket, or worn valve guides and valve stem seals. If we have to go after the internal stuff, it is gonna cost a lot more than $16.