I also want to visit friends in Singapore and maybe link the visit to a trip to Vietnam. makes sense. So as a British citizen on a 10 year SOR can i just fill in the e-visa and enter? here is what i found :

If you are a British citizen traveling to Singapore, you must obtain the Singapore Electronic Arrival Card, also known as the SG Arrival Card, to gain permission to enter the country.The Singapore Arrival Card is designed to expedite the arrival process of British travelers by avoiding time-consuming queues when they arrive in Singapore.To successfully receive a Singapore Electronic Arrival Card you must follow the Singapore SG Arrival Card requirements for British citizens.

Do UK citizens need a visa to enter Singapore?

British citizens traveling to Singapore for a stay longer than 30 days will require a visa in addition to their SG Arrival Card.If that is the case, it is recommended that they contact the Singaporean embassy in London.Nationals of the United Kingdom must note the SG Arrival Card in no way replaces the need of a visa.

I did not see any questions regarding criminal convictions. 

If you know different please comment.