Beginning with Vista, MS has integrated IE7 into the operating system (IE7 cannot be removed).
What would be the consequences of attempting to install a non-MS browser – would there be a conflict?
I have Vista Home Basic and Opera 9.5 as a default browser. There aren’t any conflicts with IE.
I have Vista Ultimate (SP1) users also using Firefox - no problems.
If there were issues it only takes a few seconds thought to imagine the multi-billion fines that would be headed Microsoft’s way in the US and in the EU.
I don’t see any consequences only benefits from installing a non-ms browser ;D even though I don’t use Vista.
I have just ordered an new system yesterday and guess what OS I asked to be installed, WinXP Pro with SP3.
No problems with a second or third or forth non-MS browser.
You can set the default one.
Here MS Vista Business 32bits and Firefox 3 as default.
I’m using Vista Ultimate SP1 and set Firefox3 as the default browser.
There aren’t any problems.
Changing the default program is a users choice. 
Trying to get rid of MS IE7 could force you to format your hard drive. Just endure it and use FF as many millions of web surfers do, it’s more secure.
There are usually no conflicts, only benefits. Just try them all and pick the browser that you like best.