There have been a number of reports of this and (I think) if affects unbranded as well as branded devices. That would point to an issue with Samsung firmware rather than Vodafone's iteration of it. Whether it affects other networks, I don't know. It seems to occur when the phone steps back from a 4G connection to 3 or 2G in order to take or make a call. This is normal as 4G is data-only. The phone should then hand back to 4G when the call is completed.
I haven't myself used speech and data at the same time, so can't say whether this can be done, but I can't see why it shouldn't be possible (in the same way that you can use broadband and talk on a landline). If it was widespread across other makes/models, I'd expect to hear more about it, so it does seem to be something specific to the S8. Is the phone dropping back to 3G or 2G, do you know? In the latter case, the connection is usually too slow to accomodate the data requirements of current apps, so it could be that they're timing out rather than failing completely. Again, this would not be manufacturer-specific, though.
Assuming Samsung are aware of this, I think all you can do is wait for an update form them.
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