Strangely enough you don't really need a function!
A = c(1, 4, 2, 3)
B = c(1, 5, 2, 9)
> A == B
[1] TRUE FALSE TRUE FALSE
You could filter using this too:
> A[A == B]
[1] 1 2
Or put your vectors in data frames:
> tibble::tibble(A = A, B = B) %>%
+ dplyr::filter(A == B)
# A tibble: 2 x 2
A B
<dbl> <dbl>
1 1 1
2 2 2