Sounds like the threads on both the MVP and iclear30s are messed up and there's not enough between the two to hold. The threads on the other tank may still be ok and is providing enough bite to hold a little - STOP using that tank on that MVP as continuing to try it will only result in messing up the threads on that tank.
You need an ego to 510 adapter - ego for battery side with 510 tank end. Most vape sites that are any good have the adapters for a few bucks.
The link Sthur gave is the opposite of what you need. That goes into the 510 threads and as yours are shot you will have the same problem. Here's the one you need.
eGo to [eGo] Adapter
Put it on the MVP ego threads and you can attach both ego and 510 tanks. Use a beauty ring to give the added support and reduce the risk of stripping the threads again.