There were still VN50 and VN54 in April a moment ago from LHR on their website.
If it is the case that Vietnam Airlines are indeed losing their LHR slots then surely this would occur at the end of the IATA winter season which ends on the last Sunday of March, which is the case every year.
The IATA winter season officially ends on the Saturday 25 March 2023 with the IATA summer season commencing the following day on the Sunday 26 March 2023.
I'm sure that Vietnam Airlines would like to remain at LHR and might be trying to do so hence why flights are still showing on their website.
Vietnam Airlines have applied for slots at STN but that's as far as we know.
ITA Airways on the other hand are still selling seats for flights from LHR to both FCO and LIN departing from the beginning of the IATA 2023 summer season. ITA Airways have recently lost their LHR slots which were leased from Etihad Airways. Etihad Airways have regained those slots and ITA Airways have made no announcement regarding their own departure from LHR.
ITA Airways could eventually come to some form of agreement with Etihad Airways or indeed another airline such as Lufthansa to remain at LHR.