Just because you're already using a vendor's ERP system doesn't mean that their data warehouse package is going to be a quick add-on. If you had to customize their ERP solution, you'll also have to customize their data warehouse package.
Remember, their data warehouse package is an ERP-centric solution. First, how much of your data is not from the ERP system? If it is a significant amount, you will need to spend a lot of resources to extend their solution to include these other data sources. Otherwise, you are merely implementing another stovepipe, in this case centered on your ERP system. The reality may be that the ERP vendor's data warehouse package may become yet another data source for your overall data integration effort.
In addition, examine the toolset the ERP vendor uses to implement their data warehousing package. Does it match you current toolset? If not, there is a cost to integrate and maintain it, and also to train your people. Does your ERP vendor understand your business intelligence needs or mainly your operational reporting requirements? If it's only the latter, you have a lot of work to do. The ERP vendor's solution may be a part of your overall solution, but is not the solution.