Well, the worst thing that can happen with such a thin mash is that you won't get conversion. That's not such a bad thing, really. I mean, you may not get fermentable sugars out of the 2 pounds of Vienna, but that's ok. The recipe will still produce a great tasting beer, even if you don't get much in the way of fermentables out of the Vienna malt. You're using about twice as much water as is recommended, so I'm not sure of the effect on the mash pH. You'll still get color and flavor from the grains, no matter what.
Do you have some Irish moss to use in the boil? If the Vienna didn't convert, you may have a bit of starch haze that may affect the clarity a bit.
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