You're trying to enter formulae into the worksheet cells with the word
'Cells' in as well as a few other variables which you may not have
defines as Names.

Try something on the lines of:
Selection.FormulaR1C1 = "=VLOOKUP(" & Cells(KeyStartRow,
TitleCol).Address(ReferenceStyle:=xlR1C1) & ",'Ref.Tables
'!C[4]:C[5],2,FALSE)"

or:
Selection.Formula = "=VLOOKUP(" & Cells(KeyStartRow,
TitleCol).Address(RowAbsolute:=False, ColumnAbsolute:=False) &
",'Ref.Tables'!D:E,2,FALSE)"

It doesn't work because in
Selection.FormulaR1C1 = "=VLOOKUP(Cells(KeyStartRow, TitleCol),'Ref.
Tables'!C[4]:C[5],2,FALSE)"

You don't want KeyStartRow. You want the current row.

This almost fixed my problem. It looks at the cell  I would like but now
instead of looking at the worksheet Ref. Tables that is in the same
workbook it looks for a totally new workbook. Can we fix that problem
too?

The same happened to me, just make sure the name of the sheet is
exactly right in the formula, spaces'n'all.

KeyStartRow and TitleCol are local variables in your program, but you are
putting them in the formula as part of the quoted static part of the
formula. I.e. the formula becomes

=VLOOKUP(Cells(KeyStartRow, TitleCol),Ref.Tables!K:L,2,FALSE)

In fact the whole "Cells(KeyStartRow, TitleCol)" is wrong. You need a cell
address. E.g.

Selection.FormulaR1C1 = "=VLOOKUP(R" & KeyStartRow & "C" & TitleCol &
",'Ref.Tables'!C[4]:C[5],2,FALSE)"

I'm not sure what you mean by 'Ref.Tables'!C[4]:C[5] either. That's a
relative reference 4 and 5 columns to the right of some unspecified position
in the Ref.Tables sheet. Do you want absolute references?