Use following instruction to load the immediate value into all fields of a vector register:
unsigned
vspltb/vsplth/vspltw
or signed
vspltisb/vspltish/vspltisw

If the value is greater than the maximum that can be coded in the instruction (5 bits provided), use vector shift instructions
vsl/vsldoi/vslo
and/or use
vor (or nvor) to combine the 2 parts.

Then add the resulting register with the other register (vadd)

Documentation is available here:
https://www.nxp.com/docs/en/reference-manual/ALTIVECPEM.pdf