1. In answer to the original question, the amount of hi viz visible from all angles on a vest is significantly reduced compared to long sleeves. That's why I prefer to use long sleeved hi viz. I have a zip up one with a velcro tab to stop the zip coming undone. Minimal flapping. Bought it from the net somewhere, but can't remember where...
Followign a name change at work, I'm hoping that the current "branded" hi-viz coats will become surplus. If I can find a way to remove the branding, I might have a source of lightweight, Gore-tex lined hi viz jackets.
2. I notice some drivers noticing me when I wear hi viz - either all over Aerostich or a long-sleeved "vest" as above - and, as a result, I feel even more vulnerable without it, especially when its gloomy. However, I think I'm grown up enough to know that not everyone is going to see me so I really don't think I ride any more carefully or defensively out of complacency - I still feel very vulnerable on the bike. It's a little condescending to suggest I somehow assume I'm ok because i'm wearing hi viz.
It's true that approaching a T-junction on the major road in the dark, the hi viz isn't visible to a car emerging from the junction at all, and the reflective stuff only reflects light back towards the light source. With a car coming out of a t-junction, that's simply no use. However, with a car turning right into the minor road and me coming the other way, hi viz is still visible and the reflective stuff can still be seen. Whether it's enough to make the driver realise it's a bike coming is another issue, but it's never going to make things more dangerous.
(The only recent time I can recall being surprised by a SMIDSY recently, I was not wearing hi viz. However, one occasion hardly makes for a scientific sample size. And I've been pulled out on loads of times, but expecting it means it's less of an issue.)
The bottom line is that it isn't ever going to do any harm or make me less visible, and if it only makes me visible to a few people who would otherwise pull out on me, it's a positive.