This is what happens (although I may be making a dumb mistake)
library(tidyverse)
tbl1 <- tibble(x1 = c("a", "b", "c"), x2 = 4:6, x3 = 7:9)
tbl1 %>%
ggplot(aes(x1, x2)) +
geom_col()

tbl1 %>%
ggplot(aes(1, 2)) +
geom_col()

So simply putting in "1" or "2" doesn't work... (neither does vars(1))