library(tidyverse)

dt <- tibble(V1=c(1,2,4), V2=c("a","a","b"), V3=c(2,3,1))

swap_if<- function (condition, val1, val2, missing = NA)
{
    if (!is.logical(condition)) {
        stop("`condition` must be logical", call. = FALSE)
    }
    out1 <- val1[rep(NA_integer_, length(condition))]
    out2 <- val2[rep(NA_integer_, length(condition))]

    out1[condition & !is.na(condition)] <- val2[condition & !is.na(condition)]
    out2[condition & !is.na(condition)] <- val1[condition & !is.na(condition)]

    out1[!condition & !is.na(condition)] <- val1[!condition & !is.na(condition)]
    out2[!condition & !is.na(condition)] <- val2[!condition & !is.na(condition)]

    tibble(out1, out2)
}

#swaps, but want to use in a mutate call
swap_if(dt$V2=="b", dt$V1, dt$V3)