Is it really necessary to properly heat oil in a pan before frying veggies?
If you are looking to fry veggies then yes, it is necessary to pre-heat.  If you put them in a cold pan with oil, you start 'sweating' the veggies instead of frying them.

For example: if you put a sliced onion into a pan with hot oil, it will cook and have a nice golden brown color as it caramelizes.  Put that same sliced onion in a cold pan with cold oil and then add heat, and the onion will first turn translucent and lose moisture.

Both of these cooking methods have their uses, but they are different.
How do you know when the oil is ready to cook? It will start to shimmer a little before it reaches the smoking point. Put the veggies in when you see this shimmer, or at the very first wisp of smoke.