Whom do I ask to borrow a dummy life vest to practise once for myself?
Ask the crew next time you fly, providing it isn't a really mega short flight they
might let you have a go. Just do be aware though that the life jackets used in the demonstration kit are (depending on type) slightly different to the real life jackets under your seat. Some demonstration jackets have a slightly larger head hole than the real ones so that the crew member demonstrating it can 1) get it on quickly 2) get it on without mucking up their hair!
Shot down in flames....why...?
I expected to be shot down in flames by the types of SLF that do not pay attention to the demonstration.
Referring to your comment about pax in front of you reading or chatting during demo, I have in the past when noisy people are stopping others from being able to hear properly (as I am the person reading the demonstration text) I have stopped reading it, stood up and looked over in the direction of the persons talking loudly. The rest of the crew demonstrating then wonder what is going on, they turn, look at me, see that I am looking at someone, then they in turn stare. Before you know it, half the aircraft is looking at the noisy persons who then after a moment or two realise that everyone is looking in their direction and feel rather sheepish. Thoroughly unprofessional I know, but you can guarantee that the next time they fly, they WILL pay attention to the crew or at the very least be quiet!