THE ACFT COLLIDED WITH THE GROUND IN IMC AFTER THE PLT REPORTED HE LOST HIS VACUUM INSTRUMENTS. THE FLT ORIGINATED AT MATTITUCK, NY WITH A DESTINATION OF LEBANON, NH. AFTER DEPARTURE THE PLT WAS CLEARED TO CLIMB TO 7000 FT. THE INSTRUMENTRATED PILOT WAS ON AN IFR FLT PLAN. ABOUT 8 MINUTES AFTER DEPARTURE THE PILOT RADIOED THAT HE HAD EXPERIENCED A LOSS OF VACUUM AND HIS ARTIFICIAL HORIZON WAS INOPERATIVE. HE ALSO STATED HE HAD NO AUTOPILOT AND REQUESTED RADAR VECTORING. HE WAS GIVEN A HEADING OF 050 FOR RADAR VECTORING TO NORWICH VOR. ABOUT 2 MINUTES LATER THE PILOT RADIOED HE WAS 'IN TROUBLE AND IN A DIVE.' ONE MIN LATER HE RADIOED 'ONE THOUSAND FT I AM IN DEEP TROUBLE HERE.' THIS WAS THE LAST RADIO CALL FROM THE PLT. WITNESSES IN THE AREA HEARD & SAW THE ACFT COME OUT OF THE CLOUDS AT 200-300 FT & THEN CLIMB BACK INTO THE FOG BANK. SHORTLY THEREAFTER, A 'WHISTLING DIVE' WAS HEARD FOLLOWED BY THE SOUND OF A CRASH. THE WX IN THAT AREA WAS DESCRIBED AS FOG, LIGHT RAIN, & OVERCAST. EXAM OF THE ACFT WRECKAGE REVEALED THAT THE VACUUM PUMP FAILED.