You do NOT have to be in VMC on your long solo x/c.
Here's the summarization of the solo requirements for the commercial from the FARs:
[ QUOTE ]
Sec. 61.129 (4)
10 hours of solo flight in a single-engine airplane on the areas of
operation listed in Sec. 61.127(b)(1) of this part, which includes at least--
(i) One cross-country flight of not less than 300 nautical miles total
distance, with landings at a minimum of three points, one of which is a
straight-line distance of at least 250 nautical miles from the original
departure point. However, if this requirement is being met in Hawaii, the
longest segment need only have a straight-line distance of at least 150
nautical miles; and
(ii) 5 hours in night VFR conditions with 10 takeoffs and 10 landings (with
each landing involving a flight in the traffic pattern) at an airport with an
operating control tower.
[/ QUOTE ]
No mention of conditions the flight must be flown in. The dual x/cs (day & night) do have to be flown in VMC, though not necessarily under VFR (you can file if you so choose)
hope that helps.
Sarah