Don't.
This is something I started to do in my campaigns. When the players derail the plot, I build a new one for them to follow. If they want to focus on how the bad guys have tech that isn't public knowledge, they can. They're ignoring the larger problem of "oh crap, zombies" while doing so, however, so simply let the rest of the world go on into decay as they mess around, until they're forced to confront the fact that the secrets being kept are perhaps a little less important than the fact that everything's collapsing around them. Let the plot equalize itself over time by letting the players do what they want; eventually they'll realize that they can expose this whole secret and conspiracy stuff, but don't have anyone left to expose it to because they've all become zombie food.
However, if you must.
Make them choose between their mission and the mission.
Eventually they have to be confronted with the following conundrum: Is our personal investigation getting in the way of the good of mankind/not being eaten by zombies? If they don't do this, then they'll continue their investigation as planned, but the world around them effectively ceases to exist as it was, meaning that they have to take responsibility for everyone dying and they no longer can get any help. If they're working with someone, have them pull support. Do so in a way that enhances the narrative, however "New York's almost overrun! Get back here or don't come back ever." works better than "and they didn't want you doing that.", because one sounds like a logical reason, and another sounds like railroading.
Dry up their leads.
If you really want them to stop searching for evidence, have the other corporation collapse. Their secrets and technology are gone with them as they crumble from within. The players could still try to investigate, but there aren't any real survivors to explain things, and anything they find essentially ends with "and now we're all gonna die".
Wave it away.
One thing it sounds like you're asking is how to explain the presence of high-tech gadgets. Simple; they're one-of-a-kind and untested. Sure they do what they're supposed to, but they're nowhere near finished. They're prototypes sent into the field. Ultimately, it's sort of your fault for introducing it if you can't explain it, but simply have it end. No more supertech gear because it didn't work, as the field trials showed. The only one got destroyed, beat up, or otherwise compromised and it proved to be less valuable than its investment.
Best Answer
The fact that it can kill any one single person without exception (from my understanding by reading a little about the adventure), the best way to prove that this stone has significance is give them a vision that the target of the stone (the count, duke or whatever) has both reasons he cannot leave the area, but also is destined to have a great importance to the world at large. And if the Stone of Slaying is found and used against him, it would stop him from fulfilling that destiny.
This way, just ignoring the stone would have world reaching consequences and can incentive them into taking the contract and finding and destroying the stone.