Voting by post or in person at an overseas polling station is meant for Singaporeans residing overseas only and does not include local Singaporeans who happen to be overseas on Polling Day, such as those on overseas vacation. 

If you are unable to vote in Singapore on Polling Day as you are overseas, your name will be removed from the Registers of Electors. You can apply to restore your name at Voter Services using your Singpass, so that you can vote at future elections.

The source of information is from the ELD website.