How to write two vectors of different length into one data frame by writing same values into same row?

One option is match

(tmp <- unique(c(ef1, ef2)))
# [1] "A1" "A2" "B0" "B1" "C1" "C2" "D1" "D2"

out <- data.frame(ef1 = ef1[match(tmp, ef1)],
                  ef2 = ef2[match(tmp, ef2)])

Result

out
#   ef1  ef2
#1   A1   A1
#2   A2   A2
#3   B0 <NA>
#4   B1 <NA>
#5   C1   C1
#6   C2   C2
#7 <NA>   D1
#8 <NA>   D2