SMA HAS INST FAILURE AND IS GIVEN 'NO-GYRO' VECTORS IN DEST.

Narrative

I DEPARTED MOR VFR INBOUND FOR TYS. THE FORECAST WX AT THE TIME OF DEP FOR TYS WAS 5500 FT OVCST; 4 MI RAIN/FOG. AFTER DEP AND PICKING UP AN IFR CLRNC; I ENCOUNTERED IFR CONDITIONS. TYS APCH CTL QUERIED ME ABOUT MY HDG BEING OFF MY ASSIGNED HDG GIVEN TO ME IN MY IFR CLRNC. I RESPONDED BY NOTING I HAD VACUUM SYS FAILURE. TYS APCH CTL INITIATED A NO-GYROSCOPE APCH SEQUENCE FOR ME TO CONTINUE THE APCH. WHILE BEING VECTORED FOR THE ILS APCH AT TYS; I WAS UNABLE TO RECEIVE THE LOC FOR THE APCH. WITH THE HELP OF ATC AND WITH STRICT RADAR VECTORS ON THE APCH COURSE; I WAS ABLE TO HAVE THE FIELD IN SIGHT AND LAND AT TYS UNDER THE EXISTING WX CONDITIONS; BUT UNDER RVR MINIMUMS. EVEN THOUGH I WAS BELOW THE APCH RVR MINIMUMS; I FELT THE DECISION UNDER THE CONDITIONS WITH THE HELP OF ATC TO LAND THE ACFT AND INSPECT FOR TROUBLE WAS THE SAFEST DECISION RATHER THAN FLY BACK INTO IFR CONDITIONS WITH INOP EQUIP ON A MISSED APCH. THE ACFT WAS GNDED UPON LNDG AND INSPECTED TO HAVE A DISCONNECTED VACUUM LINE TO THE DIRECTIONAL GYROSCOPE; WAS CONNECTED; TIGHTENED AND GND CHKED THE FOLLOWING DAY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR RECOGNIZED PROB EARLY ENOUGH TO DIVERT TO A NEARER VFR ARPT BUT THE FORECAST WX AT TYS CONVINCED HIM TO PRESS ON. THE POOR VISIBILITY AT TYS PROVED TO BE A SURPRISE. THOUGH THE RVR WAS ONLY 1800 FT; HE SAW THE RWY FROM ABOUT TFC PATTERN ALT. HE SAID HE ENCOUNTERED IMC ABOUT HALFWAY ON HIS 40 MI FLT FROM MOR TO TYS.