Also, I suppose that leads to other questions.

Could I do the same thing if the unit in Vienna were mine, rather than an Austrian unit?

And if all the units involved were mine, presumably I couldn't do it? As you can't displace your own units, right?

Yes,you can! Of course you can use it but it's questionable if "it'll work" as we don't understand what is to be working! The defense or the attack?

No you can't support others into Vienna if your unit is in Vienna.

Not true...

Your first question is yes, but why would you waste an order just to dislodge the unit (3 on 2)

Your second question is yes, bit you can't dislodge you own unit with their support, just bounce a potential attacker that way and allow your unit to attempt to move at the same time.

there is a subtlety to that last bit.

You can support an enemy unit into a territory which you have a unit in. It doesn't fail UNLESS the attack would dislodge your unit.

@ora, yes, there is some subtlety here.
I don't fully get question #1 and #2, but what I fail to see in any of the answers is this: ...

Oh, wait, does this question say: "Is supporting ..." possible? Yes, yes, yes, supporting is always possible, following the known rules (I judge you know them). I thought the guy was interested in the outcome, jeez, saves me a lot of time.
As your third question is already a derivative of an ill-defined question I can't possibly answer it.

And yes, your support hold of vienna will also work (unless vienna is ordered to move) - in case that's what you're asking.

Right, sorry, very badly-worded questions!

Thanks for the answers though.

What I meant by the first question, instead of "will it work?" was "will it succeed?", i.e. will the supported move into Vienna succeed. I gather from your answers that it will.

The second question is the same, only with my unit in Vienna instead of another country's unit. So the question is, if I'm holding my own unit in Vienna, but using two further units to support another country into Vienna, will that succeed? i.e. Can you support someone else to dislodge one of your own units?

The third question, I presume the answer is you can never dislodge your own units, right?

You can supoort them, up untul the point that your army woukd be dislodged.

Take:
Italy: A tri hold
A ven S A tri hold
F adr S A try - tri

German:
A tyr - tri
---

All the supports 'work' germany bounces 2vs2 with the holding unit in trieste.

Now add german:
A vie S A try - tri

The army in trieste would be dislodged, so the support from Adriatic is now cut, and again germany bounces 2vs2 with trieste.

Now add:
Austria
A alb S A bud - tri
A bud - tri
A (imaginery extra territiry, cause this is a bad place on the map) S A bud - tri

Now we have 3 in from austrian units, and 2 to hold. But the german attack at 3 units doesn't cause trieste to be dislodged, so the italian support works.

Germany bounce 3vs3 in trieste. (so again, when i say "it work" i don't mean the attack takes the territory)

Ps: i hope the notation for writing orders makes sense, it is fairly widespread, but i never think to ask...

So in summary to ora:
In general: status quo (no change), when attack on territory =< defense of territory, or when attack 1 on territory == attack 2 on territory. (including support, excluding cut support)
In addition to him:
If you help support an attack: status quo, when attack on territory w/o support of owner of defending unit =< defense of territory. (including support, excluding cut support)
The second part of the rule is a bit tricky to explain as it might be regarded as a rule used in exceptional situations (and is also not explained by ora for that reason, I guess). Small example (see ora's post for details on units): Around Tri, both the Austrian and the German forces combine their attack. Now, following the rules most beginners learn, 'guessing' which unit will attack will void the attack (because offensive support on a compatriot results in status quo). Because obviously, Tri is a goner; we had a very fruitful (half hour discussion) when such a situation came up in an AFK game, ah, the good old times. People still disagreed the next time we started playing.
This third part sounds weird and all, but I want to point out: the wording you learned (ie, "own support does not dislodge") is wrong.

Thanks for the explanations and clarifications - much appreciated!