My Pulsar has a VASS certificate. I required it to change the engine number after an SR20DET conversion as the engine was never an option for the car. They check engine mounts, intake system, emissions, noise, height, brakes (including a drive test with brake test) to ensure everything is safe and complies with the relevent ADR's for the year the car was manufactured.
It cannot stop you getting defected for other modifications that aren't checked, legal and listed on your certificate. You have 30 Days to submit the form to Vicroads if you have been asked to do so - in my case so I could legally register my car with the engine it had in it. I hadn't been defected, it was just a precautionary measure because there was far more on the car I didn't want an officer to look at should he realise my engine number didn't match my registration. Just a bit of piece of mind. The certificate doesn't expire as such. I'm sure if an officer was being picky your engineer can re-issue the certificate provided no changes have been made.
My certificate was $750