You can't port to vodafone paygo - they won't let you port that to vodafone contract, it will just be seen as an upgrade. You may possibly get a better price but still not as good as connecting as a new customer.
You're best bet is to port to t-mobile paygo like you say (or any other network's paygo for that matter).
You need to get your PAC code from vodafone and give it to t-mobile when you activate the paygo.
You shouldn't need to put any money on it to activate it.
Porting from vodafone to t-mobile could take upto 2 weeks. During part of this time its possible you may not be able to receive calls on your 'ported' number at all. This usually doesn't happen tho, and if it does its usually no more than a day.
The t-mobile paygo will have a temporary t-mobile number. So calling that number will connect calls to your t-mobile paygo while the porting is proceeding. Calls to your normal number will still go to your vodafone line.
After porting your ported number will terminate to your t-mobile sim, and your vodafone sim will just stop working.
Its possible somewhere in the middle your temporary t-mobile number stops working, and you're vodafone sim also stops working! But like i said usually not for more than a day - and this isnt suposed to happen.
Now, when you get your new vodafone line the process works in reverse.
The 'pitfalls' of doing it this way as you say are:
- For 2 weeks you may have to carry two phones around, not knowing which sim your calls will end up at
- There could be some period where neither your temporary t-mobile number nor your vodafone number work. You may be unreachable during this time
The above two points then work in reverse when porting back to the vodafone contract. The whole process could take upto a month. Its a big money saving, but very inconvenient. If you really must be reachable at all times, buy a second paygo and give everyone that number temporarily untill the process is complete.