iPhone battery health concerns possibly caused by VPN usage

If you have anything greater than 94%, you should be very pleased. It is expected that the average over several months is about a 1% drop per month. The drop is never linear meaning some months it will not drop at all and other months it will drop more. Calibration of the Battery Health is only possible when the battery has been allowed to charge to 100%. Some users restrict the phone from charging fully, so the calibration never happens until one day it manages to fully charge then they see a larger than expected adjustment.

As for VPN usage affecting Battery Health, that is unlikely. The same is true for using a VPN for security and privacy protections, it does not do that either.

Batteries, like people are individuals. No two people age exactly the same and batteries don’t age the same either. The exact usage, chemical makeup, storage conditions, exposure to heat and cold and about a dozen other factors mean that every battery ages differently. Battery aging is not a linear (straight line) and degrades at different rates as the chemicals in the battery age and other factors mentioned above. Apple expects your battery health to be at 80% after 24 months. If your battery health misses that expectation, contact Apple Support.

Please review the Apple Support articles on battery optimization. It contains the best  current practices for battery charging.

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

About the battery and performance of iPhone 11 and later - Apple Support

It is expected that the average over several months is about a 1% drop per month.

Not for the iPhone 15. For what its worth, here is what Apple says about the iPhone 15 battery:

Batteries of iPhone 14 models and earlier are designed to retain 80 percent of their original capacity at 500 complete charge cycles under ideal conditions.1 Batteries of iPhone 15 models are designed to retain 80 percent of their original capacity at 1000 complete charge cycles under ideal conditions.*

A link to the complete document is here: iPhone battery and performance - Apple Support

So, the iPhone 15 battery would decline in general at about half the rate of iPhone 14 and earlier models. The rub here is that this performance assumes "ideal conditions" (whatever that might be.)

Or, stated another way, the Max Capacity would decline about 1% for every 50 charge cycles for the iPhone 15.

At 6 months, the iPhone 15 battery should be at about 96-97%, pretty much in line with what the OP is reporting.

In December, I used VPN 24/7 for about a week, after which I noticed that the battery dropped to 98. Then a week later it was 1 again, and today it fell by another 1 percent

Take a look to see whether your VPN battery usage is significant by tapping on Settings > Battery and scrolling down to see Battery Usage by App.

That being said, the only valid reason to install a VPN would be if your employer or school requires it. Otherwise, your iPhone is less secure with a VPN despite the advertising hype that you are hearing about VPN.

Your iPhone’s battery health is expected to drop 1% for every 40-50 charge cycles. Keep in mind, the decline isn’t linear meaning some months you will lose more than expected, other months none at all. So it’s very possible that your battery health was steady for a couple months, then suddenly dropped a few percent.

After 6 months of your iPhone (roughly 180 charge cycles = 3.6%), as stated you should be at around 96-97%. So your battery is on track.