Vapour could be head gasket, but in this weather probably just vapour, caused by moisture in the air and residual moisture in the exhaust system, being stood over night etc.
If the head gasket has gone and is losing coolant, you'll notice one of about 4 things.
Oil in coolant.
Coolant in the oil, which causes Mayo.
Vapour even when the engine is fully up to temperature and done 20/30 or so miles.
In serious cases overheating.
However, if you have Mayo, this doesn't necessarily indicate a failed head gasket either.
Short journeys will create the stuff. If you only have a splodge on the inside of the cap, it's probably just vapour.
If you have a serious problem, the whole inside of the rocker will be affected.
It could also indicate a blocked breather hose, or breather baffle.