The first post probably needs to be updated.
Data User Plan 28 days
Top up by 20 euro to get 15 GB data and texts
Keep the 20 euro credit to use as you like
That's the plan
you need to top up within the 28 days for any reason, then top up by 5, 10 or 15 but NOT
by 20 (or more) because 20 top up will clear remaining data balance to zero and restart a new 28 days.
There are various OPTIONAL
add ons that can be purchased from credit
Add ons listed on this link
In theory you can tether without extra charges via Live APN
BUT BUT BUT
Certain phones (iPhone, Xiaomi, Android Oreo for example) allow Vodafone to reroute tethered data via another APN that charges 19c per MB
Tread carefully with tethering and check that you can edit/delete APNs to leave the LIVE APN only. If the APNs are greyed out or reappear after being deleted then it's probable that your credit will be used for tethering. Check with a small amount of tethered data and then check your credit balance to see if it has reduced or not.