"How do you protect your pills/vitamins – in a lightweight/low-bulk fashion that also maximizes the shelf life of the pill?"
For hard pills I use a very small Zip Loc baggie; for gelatin capsules I use a screw top pill vial, the ones sold here on BPL.
How long is the shelf life of a vitamin or a prescription pill, anyway? (Does it matter if it's in pill or capsule form?)
The expiration date for OTC, as mentioned is stamped on the package. Pharmacists routinely specify an expiration date of 1 year from date of filling Rx. However, I have had several docs tell me that most last at least 2 years and, if you really want to save money, ask the pharmacist at the time of filling to tell you the expiration date on the bulk container he's filling your Rx from. Example: I use Celebrex occasionally and it is a fairly pricey drug. The last 2 times I have asked the pharmacist, the actual expiration date has been more than 2 years out from filling date. Big savings given I only use perhaps 1/3 of the Rx in a year.
If you are really concerned about moisture, consider putting a dessicant packet in with your pills.
I've always just used ziplocks…but something tells me that will only work on very short trips. (I suspect the ziplock really does not protect the pills from moisture.)