Hello Marc
In October the snow cover on Vatnajökull is generally quite thin, though it is likely to be fresh snow starting to build up again.
The autumns in Iceland are generally very wet and windy with a series of low pressure systems hitting the south coast. Still there is a possibility for good days but I think 10 continuous good or ok days are going to be unlikely.
Daylight is ok, 23rd of September is the equinox, where day and night are equally long. In October the night is going to be a little bit longer than the day.
Temperature on the Vatnajökull plateau is going to be below 0°C and colder in the night, but probably around or slightly above 0 in the lowlands.
Hope this helps!