Hi Tomp, during January you can pretty much get any direction on the bay....The main wind for this time of year is a SW sea breeze, usually around the 15-20 knot range, it kicks in around 3-4pm and can blow till around 7pm. If sailing on the bay you will want to be somewhere north of Gnotuk, the more north you go along the bay (eastern side of the bay) then generally the more consistent the sea breeze will be. Can't comment on the west side.
We can also get some howling northerlies which are sailable from carrum up to Greens Point in brighton (Greens point being the popular spot).
The other wind direction we get a bit is a SE which is best sailed at either Mentone or Ricketts point (Beaumaris) on the bay. However for a SE I would recommend going for a drive down to Sandy point and hitting the waves, its usually atleast 5 knots more down there and will blow all day consistently! The direction at Sandy Pt will be cross-off which gives excellent clean DTL wave riding. Its almost 2 hours to drive to Sandy Pt.
Hopefully you will score session down at Sandy on one of the days you are here because when its on, its epic!

Here is a guide to the spots in Vic
http://www.wv.org.au/index.php?option=com_phocamaps&view=map&id=1&Itemid=77