Can you try executing these commands, in your VPS, one by one, as root.
And post the error here log if any.
# dd if=/dev/zero of=/extraswap bs=1M count=1024
# mkswap /extraswap
# echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab
# swapon -a
# echo 1 | tee /proc/sys/vm/swappiness