Hi,
see the discussion here.
With current RDF, df.Histo1D("vec[0]") is written df.Define("vec0", "vec[0]").Histo1D("vec0").
The reason you can write "vec[0]" in Filter and Define is that they take an expression as argument (either as a string or as a C++ callable), while Histo1D just takes column names. It is in principle possible to add a df.Histo1D("vec[0]") shortcut (PRs are welcome 
) but as I discuss in the discussion linked above it’s not completely trivial.
Cheers,
Enrico