Widebody; Low Wing; 3 Turbojet Eng
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness
ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification
Company Policy; Human Factors
Weather forecast to be 3000/5 at [destination] for our arrival after fog burned off an hour earlier. One IRU MEL'd inop so we were a CAT II bird.  We thought about adding gas but didn't because even though our FOD was lower than normal; we had enough fuel to do a go-around and still make our alternate. Enroute we fell slightly below our projected fuel. Approaching the point outside of the FAF; we were vectored in a 360 due to ATC spacing. By the time we got back on track and were once again a couple miles from FAF; the WX went below our 1000 RVR requirement. We diverted to our alternate. However; ATC turned us away from the field and had to be prompted that we needed to head for [the alternate] right now. I declared min fuel and said we cannot take any delays or vectors. We were vectored direct to the field. Fuel recalculation had us on deck in [the alternate] with 10.6K. Emergency is 9.0K. Approaching [the alternate]; we were given a vector away from the field. I [asked for and] received a vector direct. Landed with over emergency fuel but would not have had we accepted the vectors.  An aircraft with degraded systems and not being given enough fuel on our flight plan combined with an inaccurate weather forecast; and me not listening to my gut and asking for more fuel.The new fuel breakdown in the [company publication] seems a little smoke and mirrors. The weather was slightly above mins for the hour prior to our arrival and projected to be above 3000/5 for our arrival. I'm going to maintain my previous calculations when deciding how much fuel I will carry.
Wide body air carrier Captain reported diverting to an alternate airport after ATC vectors and deteriorating weather resulted in burning more fuel than planned.