So the important point in your question is why a new VM and not a java VM. ok let's choose a Java VM instead what do we get :
1- complex Bytecode (class) => how to store it in the blockchain for which cost?
2- useless features: Network access,I/O stream, File W/R ... => big security issues: think of it you could write a code which ping (of death) another machine or access protected file or even steal the miner keys. Even sandboxed a flaw could occurs and file write/read feature could break the whole system so think of the other features.
3- JAVA VM is a licensed Sun product, so how we will introduce the gas price in order to prevent Dos attack? how to customize the VM? to overcome this problem you need to write your own Java VM which a complex task read : http://programmers.stackexchange.com/questions/71561/java-and-jvm-license.