Wine fermentation in steel tank When fermenting wine in a steel tank, should one put on the lid or should one leave it open?

Now as I understand, if it's open to oxygen, the yeast will multiply much faster and so the fermentation could kick on really fast.

But can the yeasts still multiply if the lid is on (with a pumped up silicon tube but I left 20 cm or so air between the must and the lid)?
Your biggest worry with an open fermentor is contamination. 
If you've added potassium or sodium metabisulfide to the wine must, you should be safe against most wine spoiling bacteria.  However, it is advisable to cover the fermentation vessel.  See this post about open fermentation.