R equal sampling takes too long - r

Question

I want to sample rows from different years given some constraints.
Say my dataset looks like this:
library(data.table)
dataset = data.table(ID=sample(1:21), Vintage=c(1989:1998, 1989:1998, 1992), Region.Focus=c("Europe", "US", "Asia"))
> dataset
ID Vintage Region.Focus
1: 7 1989 Europe
2: 10 1990 US
3: 20 1991 Asia
4: 18 1992 Europe
5: 4 1993 US
6: 17 1994 Asia
7: 13 1995 Europe
8: 9 1996 US
9: 12 1997 Asia
10: 3 1998 Europe
11: 11 1989 US
12: 14 1990 Asia
13: 8 1991 Europe
14: 16 1992 US
15: 19 1993 Asia
16: 1 1994 Europe
17: 5 1995 US
18: 15 1996 Asia
19: 6 1997 Europe
20: 21 1998 US
21: 2 1992 Asia
ID Vintage Region.Focus
I want to 1,000 draws of sample size 2 and 4 (separate from each other) spread along two years. E.g. for 1,000 draws of sample size 2, it could be the first and the second row. I also have a constraint that the sample must consist of rows with the same region focus. My solution is the code below, but it is way too slow.
for(i in c(2,4)) {
simulate <- function(i) {
repeat{
start <- dataset[sample(nrow(dataset), 1, replace=TRUE),]
t <- start$Vintage:(start$Vintage + 1)
matches <- which(dataset$Vintage %in% t & dataset$Region.Focus == start$Region.Focus) #constraints
DT <- dataset[matches,]
DT <- as.data.table(DT)
x <- DT[,.SD[sample(.N,min(.N,i/length(t)))],by = Vintage]
if(nrow(x) ==i) {
x <- as.data.frame(x)
x <- x %>% mutate(EqualWeight = 1 / i) %>% mutate(RandomWeight = prop.table(runif(i)))
x <- ungroup(x)
return(x)
} else {
x <- 0
}
}
}
#now replicate the expression 1000 times
r <- replicate(1000, simulate(i), simplify=FALSE)
r <- rbindlist(r, idcol="draw")
f <- as.data.frame(r)
write.csv(p, file=paste("Performance.fof.5", i, "csv", sep="."))
fof <- paste("fof.5", i, sep = ".")
assign(fof, f)
}
This code is very slow. My initial intuition is that my approach would need a lot of funds and keeps looping due to the constraint. I have 5,800 rows.
Is there a way other than the repeat function that results in a lot of looping? Perhaps there is another way of expressing the line DT[,.SD[sample(.N,min(.N,i/length(t)))],by = Vintage] to get rid off the repeat expression? Thank you in advance for any input!