All the keyboard will transport via midi, is midi. No audio. You'll have to plug into the harmonizer for that. But I think you already know that.
You will probably need to be sure the channel transmitting chord or note info from the keyboard matches the input channel of the harmonizer. Most are fairly automatic these days, but you may have to set whether the kb transmits below or above the split, or both.
This setting is generally made via instructions from the harmonizer, but touching a key below or above the split.
Also, be certain the harmonizer is set to not receive any program change or control change information, or else be sure the keyboard is not transmitting it. It will make changes to your effects, and maybe other things you don't want to happen.
Again, with the VK and the Harmony M, you can make this setting from the harmonizer quite easily. You'll have to read the manual on yours to see how to do it.
You will need to use a midi Through on either the harmonizer or tablet to use both from the same connection. I don't recall whether the PA900 has two midi outs, but I think it does not. There may also be a midi merge or split device available but I'm not familiar with those so can't help there.