Do you really need to convert back again ? If you are converting them from Lat/Long in the first place why not maintain a hash (or tied db if too many for hash) keyed on V&H of each Lat/Long you convert ? This assumes you are not performing any transformations of the resultant V&H only calculating various mileage distances as given above.

The other possibility is to calculate mileage directly from the Lat/Long pairs, this is not too hard (I can't remember the formula now but look up spherical geometry or celestial navigation (as in with a sextant, not for spacecraft :-) You can solve a spherical triangle based on the angle your two points make with one of the poles and the distance between each point and the pole (90-Lat)*CircEarth/360(in your desired units).

# why is there not a <pseudocode> tag :) ?
# here is the triangle we solve...
#
#     A (North Pole)
#    / \
#   c   b
#  /     \
# B---a---C
#
# Point A is the north pole, B an C are all yours
# $pointB $latB, $longB
# $pointC $latC, $longC
# here is the formula
# cos a = cos b * cos c + sin b * sin c + cos A

$miles_per_degree=$Earths_Circumference/360;
$Angle_A=abs($longB - $longC); # needs fix for wrapping round the prim
+e meridian
$Len_b = (90-LongC)*$miles_per_degree; # fix for south of equator
$Len_c = (90-LongB)*$miles_per_degree; # make degrees S negative ?

# plug those three into the spherical circle equasion
[download]

Cheers,
R.