Then shouldn't it work in reverse for items that increase hp?

<100/200hp> wear brown;wear brown
You wear a sandy brown cloak.
<120/200hp>
You wear a sandy brown cloak.
<140/200hp>

Seems like what's happening with the vis'pas is the above in reverse, e.g.

<100/200hp> rem necklace;rem necklace
You remove a necklace of vis'pa.
<100/200hp>
You remove a necklace of vis'pa.
<100/200hp> wear necklace;wear necklace
You wear a necklace of vis'pa.
<50/200hp>
You wear a necklace of vis'pa.
<0/200hp>