You've done well to get 2 years out of it with no problems!!

Glow plugs are not needed to start a vehicle these days and are used as an emissions device to heat up the combustion temperature on cold start/running. They are only useful as a starting device when you are in a cold climate.

You cannot mix and match different makes of glow plugs (I.E. if you have Renault plugs fitted then fit NGK plugs the fault code won't go away). Usually a glow plug fault produces P0380 fault code.

There is a glow plug controller somewhere (I can't remember where they are on Vivaro/Trafics).

Now that I have typed all that out, I'll tell you what I think it is...

On Renaults and Vauxhall vans, they have EGR issues but not usually on the Vivaro/Trafic unless it's got an electric valve. Disconnecting the valve usually cures this for some reason. Replacing the valve doesn't help. With the valve connected, the engine will start and cut out a few times, then when it does start and idle, it won't rev until it's ran for a while. Then it's ok. Disconnecting the valve stops this issue and they start and run and rev straight away. However I do not think you're suffering this issue.

The issue that I think you will be suffering from however is carbonisation of the piston rings. Carbon builds up around the rings and they stick. The engine can be bad to start and then when it does stay running, it'll only idle and won't rev up. The best thing you can do to try and resolve this is use a strong engine flush. We use a flush that is not available to the general public but anything is worth trying. If you can find it, Forte or Sol-X is good stuff. Get the flush and pour it into the engine. Get it running and leave it running for an hour or so. Let the cooling fans kick in a few times and leave the engine idling while you have the flush in. Do not rev the engine. Once this process has been done, drop the oil and filter and replace with new. This usually is enough to break up the carbon and free off the piston rings. Extreme cases need an engine strip down.