The VSENSE0~3 have an internal voltage divider with divides the respective input voltage to a 1V output level.
E.g. VSENSE0 has a 10:1 divider. Those internal restrictive dividers have about 800kOhm with a wide variation over products and temperature (about 20% between devices, about 15% over temperature). This resistance will be causing errors in your external divider (Rinternal || Rdivider_bottom).
The better option would be to use the GPIO0~4 = VSENSE_EXT inputs for this purpose. The internal impedance is here much higher and will cause less interference / error. It's about 30MOhm (also with variations). The VSENSE_EXT inputs can measure between 0.0 - 1.0V.
If you need an operational amplifier will depend on your exact system (resistance of ext. divider) and magnitude of error which is caused be internal resistance. The device to device variation introduced error can typically be handled by an individual (per ECU) system calibration. The temperature variation introduced error needs to be evaluated.
