[name_m]Welcome[/name_m]!
There’s an ever so slight rhyminess between [name_f]Vivienne[/name_f] and [name_m]William[/name_m] which would push me in the direction of [name_m]James[/name_m] for you. They have a similar rhythm to them, two short i-sounds then the softer ending.
Having said that I like that [name_m]William[/name_m] shortens to [name_m]Will[/name_m], so both kids have a nickname. Maybe you could find a nickname you like for [name_m]James[/name_m]? I like [name_m]Jem[/name_m] best, but you could always go with the classic [name_m]Jim[/name_m], [name_m]Jamie[/name_m], or wild card, if you call him [name_m]James[/name_m] [name_m]Oscar[/name_m] maybe [name_m]Joss[/name_m]!
Good luck!