Dear Klaus,
the VCOREMON automatically sets to the BUCK1 or BUCK1/2 in dual phase output voltage. You just need to set the BUCK1 output voltage. The VCOREMON overvoltage and undervoltage thresholds are listed in Table 57. in the VR5510 datasheet.

The output voltage is configurable by OTP through the BUCK1V_OTP [7:0] bit field (CFG_ BUCK1_1_OTPregister) or the BUCK2V_OTP [7:0] bit field (CFG_ BUCK2_ 1_OTP register) from 0.4 V
to 1.8 V.
Please refer to section 23.6.2 in the VR5510 datasheet for the BUCK1/2 output voltage transition.
With Best Regards,
Jozef