Actually, the Soviets allready had bridgeheads across the Vistula.
In the wake of the successful
Operation Bagration, the
1st Belorussian Front managed to secure two
bridgeheads west of the Vistula river between 27 July and 4 August 1944.
[4] The Soviet forces remained inactive during the failed
Warsaw uprising that started on 1 August, though their frontline was not far from the insurgents. The
1st Ukrainian Front captured an additional large bridgehead at
Sandomierz (known as the Baranow bridgehead in German accounts), some 200 km south of Warsaw, during the
Lvov–Sandomierz Offensive
https://en.wikipedia.org/wiki/Vistula–Oder_Offensive
The reason for not helping the Armia Krajowa was political.
The Armia Krajowa was all that stood in the way of Stalin controlling Poland after the war. If the Germans took care of this problem for him, the better.
Therefore, no intervention.