Re: VNX5400 Storage Pool Design
Given exactly what you have I believe I would consider the following RAID 5 configurations:
Leaving the four vault disks and one hot spare gives you 25 SAS disks. You can use them as a total of 5 x (4+1R5) in one or more pools.
Your 20 SSDs can be used as 2 x (8+1R5) with two left over as hot spares, or if you are making two pools, you could use 2 x (4+1R5) in one, and 8+1R5 in the other with one hot spare left over.
One big pool would be simple, but two would permit you to separate things from each other. You should give due consideration to this decision, as it will be inconvenient to adjust later on.
You should also be aware that a pool does not function well when it is close to 100% full. Conservatively, I use ~85-90% full as a rule of thumb; you could probably go a bit higher than that without encountering any adverse behaviors.
The documents linked above are good references; you should look them over.