The reason for the VAT (in the USSR it was called a Turnover Tax. They're the same thing but a VAT does not tax means of production) was how incredibly simple it was to administer. Tax fraud could be kept relatively low for any given level of administration if a VAT was used instead of an income tax, for example. Many modern governments do this today for the same reason.

The USSR was constantly in a struggle to build an effective administration (a good book on this is Origins of the Great Purges ) while fighting what we today would call Bullshit Jobs and local strongmen.

Post-Deng China basically took a hands off approach to local strongmen and corruption skyrocketed but economic growth was heightened. The USSR was trying to mitigate markets (most of which were black markets) to prevent shortages and mismanagement in its planned economy. So the Deng approach was not something it could do. The USSR was much more developed at the time and the intricate web of an economy was more complicated than in China.