The cable may not get fixed for another 4 weeks.

All the reports i've read are that Vava'u was not as severely impacted. Of course, nothing is for sure until the cable is repaired and communication established.
The two Niuas should be okay because of how far they are. I served in Niueatoputapu not too long after it was hit by a tsunami and the damage the island took was severe. I keep thinking about how the recent tsunami is going to be like that but on a much larger scale.

I have seen some of the aerial photos that the NZ recon plane took and Nomuka, Fonoi, and Atata looked the worst. All the buildings but one were destroyed on Atata. The crops on a lot of the Ha'apai islands were destroyed so a food shortage is definitely a concern now.