Does anyone know whether we would still need to include ImageVault as part of the CMS 6 -> 7.5 upgrade? Trying to save costs and seeing as though we are not using it much is it possible that we remove the product?
ImageVault 3 won't work in EPi 7 or newer.
To keep ImageVault in the solution you need to upgrade ImageVault to ImageVault 4.
As Johan pointed out you need to upgrade to ImageVault 4 if you are going to EPiServer CMS 7.5 and want to continue using ImageVault. I believe you can actually save costs by using ImageVault, but that of course depends on your requirements. ImageVault 4 is a much more versatile product and could probably help solving other need regarding Media Asset Management within your organisation.
Please do get back to me if you want more information regarding the benefits of using ImageVault or if you have any questions regarding migration that is not answerd by http://imagevault.se/en/documentation/api-documentation/?page=migration/index.html and http://blog.imagevault.se/post/103117799982/automating-imagevault-migration.