Is it acceptable to issue an employee in Singapore who is underperforming with a verbal warning, if his/her performance does not improve then a written warning can be issued?After an agreed timeframe if no improvement is seen then a final warning can be given possibly leading to dismissal of employment?

Yes, the above statement you mentioned is perfectly right.

However, you may want to take note the following:-
1) All warning letters must be properly filed, if possible to get the employee signature to endorse on it, if not, is still fine.

2) For verbal warning, it is good to document down, the date, venue, and the content of the discussion.

3) If possible, try to quantify the performance, if not, is ok.

4) If financially allowed, you may want to terminate the employee services instead of dismiss the employee, this will help to avoid unncessary issues.