Normally both companies have calculators which can be used to estimate this, but real estimation heavily depends on data you use and what dedupe levels will you achieve with Veeam combo.  Reach out to either Veeam or DD support and they should be able to provide tooling that might do estimate.

Once you get the Veeam sizing - go buy DPS for VMware. It's cheaper and only needs half the space as Veeam with your existing Data Domain.