I reckon Merck timed the takeover of VLA shortly after BMS did the Nektar deal to reduce the chance that BMS bids on Viralytics. BMS is probably trying to bed the Nektar deal down so is unlikely to then also bid on Viralytics. That is my thinking.
IMO other reasons include:
*Lepu taking a stake meaning VLA is cashed up to fund the trials on its own
*VLA trading at its 52 week lows, take it over before it rises on soon to be released trial results
Outside of BMS, not sure who else is going to stump a premium which exceeds what Merck offered. BMS could do it to make Merck pay more but I can't see someone else taking the risk that they (instead of Merck) win it. Maybe Lepu (as I'm sure they know quite a bit about Cavatak in taking a 13% stake) might do a counter but they are in favour of Merck's deal. Only thing is they are a $6.6B company which means making an offer than exceeds Merck's would be a decent chuck of their market cap. Unlikely.