So much effort went into keeping East Germany in the Eastern Block it seems strange the Soviets would just hand over thier section of Austria in exchange for vague promises of neutrality. Were they not at least worried it would provide, in the event of a war, an easy southern vector of invasion deep into Czechoslovakia and Hungary? Was it later considered a mistake or temporary lapse in judgement?
This has come up a few times recently, and you can find previous answers here:
http://www.reddit.com/r/AskHistorians/comments/1stom4/how_did_austria_manage_to_not_be_split_like/
The gist of it is that after Stalin's death the new Soviet leadership was more up for coming to accommodations with the West. A neutral Austria benefitted both parties and an offer of Soviet withdrawal was a useful bargaining chip.
I'm not sure why you consider the promises of neutrality "vague", though, considering that it was (and still is) written into the Austrian constitution, alongside strict restrictions on Austria's military. For example, under the treaty, Austria agreed to have no guided missile systems of any kind, meaning that until as late as the early 1990s, Austria's air defence was conducted by elderly Saab Draken aircraft with only cannon armament. The implosion of Yugoslavia from 1991 and several instances of Yugoslav MiGs violating Austrian airspace with absolutely nothing the Austrians could do about it led to the repealing of that particular law and the acquisition of some Sidewinders.