not to be silly about two quick VDD&VSS questions

[sparkydonkey]

I correctly assume that its always a symmetric ratio so if your MCU had two VDD pins then theres supposed to be two VSS pins equally as well, right?

and likewise to that I assume that even if the first VDD pin was for 2.5V but the second one was for 3.3V, the two VSS nevertheless don't care which is which?

[aeg]

Especially with MCUs now often having separate Vcore and Vio supply pins but modest power consumption and low pin count packages, there may be a single ground pin but multiple supply pins.

Generally all grounds will be connected to the substrate and should be at the same potential.

[pcprogrammer]

aeg beat me to the punch, but I present valid examples. 

I don't think your first assumption is true. For instance the Allwinner F1C100s has multiple VCC and VDD pins, but only a single ground pad. Same for the T113-S3 and T113-S4. I'm sure there will be plenty more examples of MCU's having a different number off supply pins versus ground pins.

The second assumption is valid though. Ground (VSS) is the 0V potential and carries all the current from the supply voltages back to the supply.

[Xiamatsu]

Most often, yes, paired conclusions are made side by side, most likely for the convenience of wiring the installation of filter capacitors.
For large chips with a large number of pins (BGA for example)  It's different there.