IF someone asks to borrow my SIM card, am I vulnerable? YES
Your sim card is like your social security number for your cell phone. Lending that to someone is the same as giving them your social. Now they have your information, you identity on the cell network, and can rack up charges by impersonating you. Worse yet they can even intercept your data.
A sim card, like any other storage medium can be recreated 1 for 1. That means for literally all purposes it is YOUR sim card. This means that they now have access to your entire cell data history, allotment, settings, calls, texts, everything.
If I borrow someone else SIM card, am I vulnerable? ... Maybe?
What do you mean maybe?
Well... really it's more like yes. A SIM card is usually a storage medium, but if it looks like a SIM, talks like a SIM, but isn't a SIM then it's a possible attack vector. These devices can send any data/commands sent to them out, and can send commands and other things to your phone in a way that could cause problems if a vulnerability exists.
What about if I trust the person who owns the SIM card?
To... what? Trusting someone who isn't you in security matters is like saying "You look like I think you should, here have a copy of my social security number." That sounds a little dangerous to more than just me. This is why I said maybe. If you "know" they are trustworthy and are willing to risk it, who are we to stop you?
I still would never do this. If I need to check my device, I would go to a dealer that is trusted, or a service center that is trusted. Those people have life impacting incentives to make sure they keep trusted and on the up and up.