The benefit of upgrading a SIM only direct through Vodafone @Toonlad is unless things have changed Vodafone will write of what remains of the SIM only contract after when taking out an Evo contract, you need to have had the contract for 6 months and you should qualify. The best way to find this information is from your online account, or from the Application, MY account, my plan. There is further information on Evo here: Evo
If you go the third party route although you will be paying less for the contract on a monthly basis, you won't meet the criteria for upgrading early without buying yourself out.
To answer your question about the S10, if your daughter is on Vodafone there won't be any problems. If she is on an alternative network and the phone came direct from Vodafone it will be more than likely locked to Vodafone. You can get an unlock code here: Unlock My phone , some Samsung phones need a Master code to bring up the unlocking menu and this seems to be something Vodafone fail to give, if you need a MSK Master Code, come back to the Community and the Social Team will make sure you are provided with the code.
Other than that, contracts are airtime only and you are free to use the SIM in any phone you wish.
