horizontal lookup/fill

replace the column number with
COLUMNS($A$1:A1) (thats = to 1)
columns simply counts the cols in a given range when dragged across it will change to
COLUMNS($A$1:B1) Ie 2
COLUMNS($A$1:C1) ie 3
so in your case starting from 2 use
COLUMNS($A$1:B1)

vlookup($A1,sheet1!$A:$D,COLUMNS($A$1:B1),0)
note you dont have to use the columns you're in

COLUMNS($BA$1:BA1) for example would also = 1 no matter where you use it on the sheet
you can also do the same with rows() on dragging down
rows($A$1:A1) =1
changes to
rows($A$1:A2) =2