Changing high pressure ac hose on my car, is vacuuming absolutely necessary?

As soon as you crack open the line the humidity is in. Just take it to a shop and they will do a pump down recharge for 100 bucks after you swap out the line. Otherwise you are wasting your time. The machines they have to do this are absolutely necessary to get the moisture out and check your connections and put the exact amount back in. Trust me.

It really needs to be charged to the exact proper amount but also with the exact proper pressure. Charging without the vacuum creates extra pressure. I have diagnosed many poor performing a/c systems as overpressurized sometimes with excess freon sometimes just with excess air. Sometimes I just evacuate, vacuum and recharge with the same amout of freon I took out to make the repair. Moisture and air obviously don't belong in the system. A/C requires perfection, shortcuts just don't work out. The minimum outcome is a slight loss of cooling, maybe 5 degrees, and excessive high side pressure which shortens the life of the compressor. The smaller the system the more critical it becomes. There are some cheap air operated and electric vacuum pumps, less than $100, check ebay. Buying the tools is just part of fixing cars. How will you weight the proper amount of freon? Just one extra ounce can make a small system work horribly.

Yes. You need to draw (and hold) a vacuum to eliminate moisture from the system and test for leaks.

You're supposed to evacuate the system first. Not just take the line off and dump all the 134a into the atmosphere. When a system is completely empty you need to pull a vacuum on the system for at least 20 minutes before refilling the system.
You can just add 134a if the system still has pressure in it.

Fix your car properly. Spend the money and get the system evacuated.

Agree with the other posts, you need to have a vacuum pulled on the system.

Yes it is

Vacuuming is necessary to remove all air and moisture. If you're willing to spend $100, just have the ac tech to the job right and recharge with the proper load of gas and lube after you install the new hose.

You should, but if your not going to you should try and purge the lines of all the air by pushing Freon all the way through the system. Start on the low side and find a place on the high side let the air out.

You can allow some gas to escape on one end while adding from the other, this is like flushing and would displace any air in the system. If you don't leave it open for long and it is in good condition and didn't lose much oil it will be fine.

How much is the compressor, tubing, hoses, and condenser? If it is all a lot less than $100, you can re-charge it without removing the moisture and when you have to replace all of that stuff, rent or buy a vacuum pump and do it right the second time.