Currently (as an example) vodafone offfer monthly price plans on their website with nice round numbers inclusive of VAT (£15,£20, £25 etc..). If I was to get a new plan from them now my bill would go up in January to include the new 20% vat rate. However post the 20% VAT increase will vodafone (or any other operator) be advertising their plans at say £25.24 to new custoemrs or will they have £25 plans that include the new 20% VAT rate.

So essentially is best to wait for the VAT increase to get a new monthly plan or just get it now in December?

I would assume it would be as when the VAT went down, the advertised cost fell as well.

I would not envisage any network will swallow the VAT rise for New Customers.

My guess is that the headline price will go up. Then about spring time there will be new contract launches with rounder price points.
(March is when O2 are adjusting bolt-on prices, so I guess somewhere around there)

Your line rental price will go up.

I have checked with T-Mobile and they say that if it goes up, your line rental goes up aswell.

Maybe the price points will stay the same for new customers, but the number of minutes/texts will drop.

I think you've misunderstood the point of the thread. The OP is asking what the price points will be for NEW customers after the VAT increase; not what will happen to existing line rental.

For example, will a tariff that's currently being advertised at £15 a month be advertised at £15.32 a month?

Both brands of Everything Everywhere (Orange and T-Mobile) will be increasing their price plans in respect of the VAT change. New instore POS material is being delivered for the former last week in December....