I can relly recommend proxmox. Some years ago we switched from a 60.000€ dell VMWare Storage/Server-Setup to a three Host proxmox Setup for about half the price (to be fair, add 5-10k for Setup for our local Linux Team because we did not know much about proxmox). Mainly because we were able to place one of the Hosts in our Warehouse (connected with 10g Fiber) so there theoretically will be no harm to our production in case of water/fire/whatever in the server room because the one system can instantly take over (after some learning it works Like a Charm). I had some concerns regarding ceph, but for us it has proven Rocksolid, even while we had some real weird Switch issues it always recovered fast and without issues as soon as the connection was there. A big issue were the licensing terms for Microsoft products because with three amd-systems you have a lot of cores to buy licenses for - so we had a good excuse to substitute and cut out some products that only supported Windows environments.

I like Virtualbox, use it myself in several instances but I would never consider it a replacement for VMware.