Thanks for bringing that up. It's interesting information, since abilities and buffs are not interrupted by viewing vistas. They're consumed and expired without interference by the vista in-game footage.
One other thing came to mind after your post and I couldn't help it but to go test it: falling damage during a vista footage. I went to the Lion's Arch pole vista to test it. It soon became clear you can't jump, move or dodge first and then press "F" to initiate the in-game footage, since it "F" key is only available if you're immobile. Any type of movement suppresses your ability to initiate the vista.
So I tried the next option: "F" first and jump last. And guess what... it worked. I viewed the vista footage without my toon at the pole. Vista done, I find myself lying on the ground, dead, with a bunch of people rezzing me.
Something still felt amiss after that death by falling. Falling damage in MMOs is usually calculated as a percentage of the total life the character has, so no matter if you have a thousand or a million health maximum, you'll always lose a percentage of that based on the height you fell from. Any kind of gear damage reduction is also irrelevant for falling damage. Calculating damage this way is done so early and later levels, well geared and poorly geared toons, heavy armour wearers and squishies, all are subject to the same chances of survival from falling damage.
So, if the vista footage was what had actually prevented damage from mobs, it would have done so by interrupting the normal damage calculations done by the game. Should that be the case, immunity to damage should have worked in both mobs and falling types of damage. Since that was not the case, I figured something else must have prevented mob damage. Some more investigation was in order.
I then went to another vista, outside Lion's Arch, and here's what I found.
Try #1: I attacked a couple bats and run to the vista, waited until the bats started attacking me and pressed "F". Nothing happened. Even
though I could still see the "View [F]" option on my screen, the footage would not start. At this point I already suspected what was the solution to our problem. But a scientist must do things analytically, so more tries were in order.
Try #2: I run to the bats, got their aggro by proximity, without attacking, run back to the vista and waited to be attacked before pressing "F". Again nothing happened. It was already clear what the answer was. Just needed one more try to confirm it.
Try #3: I run to the bats, got their aggro by proximity, without attacking, run back to the vista, waited for the bats to get close, and pressed "F" before they hit me. Vista footage started normally. As soon as the footage ended the bats were still by my side, attacking me and I had not taken a point of damage.
The conclusion seems pretty obvious in hindsight. Vistas do not grant immunity to damage. They in fact interrupt the character being flagged as "in-combat". If you manage to start the vista footage before getting combat flagged, you'll be able to see the vista without being hit, since flagging becomes interrupted during the footage.
That's why falling damage immunity doesn't work, for that type of damage is independent of being combat flagged.
Also, what prevented me from starting the vista footage during the first two tries was being combat flagged, meaning vista footage and combat flagging prevent the start of one another.
Considering the situation Frel described, a ton o mobs chasing after a player, I wanted to know if abilities like aegis and retaliation, which prevent you from taking damage, would also work in preventing combat flagging from mobs. The short answer is no! Even though they do prevent any damage taken, by blocking the attack or reflecting back the damage, these two abilities I tested do not prevent combat flagging.
Your best bet not to flag while having a ton of mobs chasing you is to keep your back to them and not attack or auto-attack, run as fast as you can, and press "F" quickly.
The guardian shout "Retreat!" is great for a vista sprint. Mine gives +33% Movement Speed for 26s. Pretty good for a run for it. The staff ability "Symbol of Swiftness", however, is not a good bet if you have mobs already on your tail, even for a short vista sprint, since it causes damage and will probably flag you as in combat, preventing you from starting the vista footage.
My two pence on the subject. Please forgive any typos, I'm all thumbs.
:)