That is definitely a valid txt message from Vodafone. Has the plan changed now for you? if not it is best to contact the number in that txt message 0800 434 448 (ext 238) as they will be able to answer any question if this has not been amended yet between 8:30am - 5pm.
I received this text from ( +64 21010038225) last Friday and wondered if it's a scam:
Hi there, as a loyal customer of Vodafone we have reviewed your prepaid account and have an exclusive Vodafone loyalty offer! It includes 200 mins to landlines & mobiles (NZ & AUS) & 1GB data and extra 500 MB data bonus for just $24.99 per month open term (cancel anytime, no contraracts). If you are interested reply "YES" or call 0800 434 448 (ext 238) between 8.30am-5pm.
I replied YES but nothing changed on 'My Vodafone' then this morning I got another text from another number ( +64 21010038226) saying:
Hi, I just tried to call you in regards to a plan I mentioed earlier. Unfortunately I have been unable to reach you. Is there perhaps a more suitable time between 8.30am-4.45pm I could try call you again? Thanks Payal