You need to talk with Axiom about the sound pressure level you want to achieve with the VP180 and the frequency you are planning to cross it over to your sub.

If you are to believe the specs on the VP180 page, you should be able to drive it to around 115dB before compression takes place. Likely however, compression will start before that on the low end. If you are crossing over to a sub at around 40Hz or more, you will not need the HP in my opinion.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated