Pretty simple -
Just add up each bar's closing price times its volume in the MA's length and divide the total by the total volume in the MA's length and then divide that by the # of bars in the MA's length
Something like this (don't use/like EasyLanguage - but you should be able to adapt as necessary):
Function VolWeightMA(n as long) as double 'where n=# of periods in MA
dim VW as double, V as double
VW=0 'Used to aggregate Volume*Price
V=0 'Used to aggregate Volume
for i=lastbar to lastbar-n+1 step -1 'lastbar=# of last bar in series
VW=VW+Volume(i)*ClosePrice(i) 'i=bar #
V=V+Volume(i)
next i
VolWeightMA=(VW/V)/n
End Function
'Note for smplicity of notation Volume(n)=volume of the Nth bar and ClosePrice(n)=closing price of the Nth bar