We don't have the vaccine here in the US, so I'm not sure how long you need to wait.
If it was white mucous, it would for sure be a bacterial infection causing the sneezing and discharge. Clear mucous isn't as sure an indicator of a bacterial infection, but it can still mean that this is what your rabbit has. And since you have changed hay and litter and it still hasn't made a difference, it makes an upper respiratory infection a more likely cause for it. Other possible irritants that could cause sneezing is excessive dust from something, strong household cleaners, or strong scents from things like perfume and other scented items. So you could try eliminating those to see if it makes a difference, bit if not and your rabbit is still sneezing, you will likely need to treat for an upper respiratory infection with the right antibiotic.
An upper respiratory infection can be difficult to treat and usually needs several weeks of the appropriate antibiotic. The anti inflammatory meloxicam is also usually given to reduce nasal inflammation, and the mucolytic bisolvon(if available in your country) is sometimes prescribed to thin the mucous secretions.
http://www.medirabbit.com/EN/Respiratory/Bacterial/URI.htm
http://www.medirabbit.com/Safe_medication/Safe_drugs_main.htm
With none of your vets raising the possibility of an upper respiratory infection, it sounds like they aren't very experienced with rabbits, as this is a pretty common cause for nasal discharge and sneezing. Allergies are a less likely cause for sneezing and discharge in rabbits. And I would be concerned about a vet treating my rabbits, if they aren't very experienced with them, as rabbits are a highly specialized animal to treat, and a regular cat and dog vet just wouldn't know enough to give the proper treatment, and may even give a treatment that might be dangerous to a rabbit. If you can find a rabbit specialist to take your rabbit to, that would be the best thing, as your rabbit likely needs to be treated with antibiotics, and it wouldn't be good if an inexperienced vet gave the wrong antibiotic, as some antibiotics can be fatal if given to rabbits. I'm not sure though, how long you will need to wait after the vaccination was given, before you can start antibiotics. You also need to be careful when antibiotics are given, as it can sometimes cause a rabbit to stop eating, and you would need to watch out for this and change the antibiotic to one better tolerated, as well syringe feeding if your rabbit did stop eating. If you don't have a rabbit specialist near you, maybe your vet could consult with one?
If antibiotics don't help, it is possible it is due to an allergy, it's just not very common. I believe in the UK, the allergy med Piriton(chlorpheniramine), has been used to treat rabbits with allergies, so maybe that is something that could be tried if you have this med available in your country.
