Thanks for both of you for the answer! Those helped me to get started with NVIC in general.
However it seems that void USBPhyHw::init() inside USBPhy_Maxim.cpp file already utilizes NVIC_SetVector function to register the _usbisr function to the USB interrupt. Furthermore _usbisr is defined as private.
If i am right(?) an NVIC interrupt can have only one interrupt handler function registered. If that is true, then i just can not run also a second user-defined function in case an USB interrupt occurs. So it seems i can not make any use of the USB_DEV_INTEN.vbus register bit of the MAX32630 chip.
Turns out the MAX32630FTHR has an onboard MAX14690 PMIC which has a register bit that keeps track if USB power is detected AND if that is a valid USB voltage. The PMIC can also generate an interrupt connected to P3_7 of the board, however the state of this pin does not change even if the register value of the PMIC updates when i plug/unplug the cable. So for some reason also this way does not seem to work.
Found a third possible way here. However the MAX32630FTHR’s pin definition does not contain any name for the VBUS pin that is located on the board…so i just can not make that an interrupt source…
Seems like i need to stick with the polling variant. 