As for legality, if it were so legal then the UK courts would not have ordered a block on First Row Sports.As the UK courts have not ordered a block on Vipbox then it is not illegal.
With my law student hat on, I'd suggest it is necessary to define "illegal" in the context you are using it. The legality of an act is determined by applying the relevant law to the facts, and the absence of a relevant court ruling does not absolve you of possible liability.
If there was an injunction against you ordering you not to use a particular service, breaking that injunction is contempt of court, with penalties for an individual including imprisonment. In the absence of such an injunction (which you would know about, and so far injunctions relating to blocks requested by rights holders have applied only to certain ISPs), there is likely no criminal liability involved in using a service that infringes the copyright of rights holders.
However, the lack of a relevant injunction doesn't mean you are free to use an infringing service without risk of penalty. Civil penalties may apply, especially if you knew or ought reasonably to have known that the service infringes the copyright of the rights holders. It is arguable that being so much cheaper than the 'official' service amounts to 'ought to have known'. (N.B. I'm no expert on copyright law, but this is the general position in relation to civil wrongs, known as torts).
The copyright position may not be clear if the infringing service holds foreign rights, as was seen in the case of pubs showing Premier League matches using viewing cards sold with the consent of a rights holder elsewhere in the EU. It was held in the courts that EU laws on free movement permitted cross-border sales of such a service. There might have been a subsequent appeal against this ruling, which I've missed.