Keep an eye on how they can

Probably not, because I don't get what you're trying to imply.
Do you mean they're somehow consorting to cheat the company?

Or are you talking about street vendors?

In the US, the term "vendors" was frequently used to describe independent contractors who provided some service to a manufacturing company.
Of course we don't have any manufacturing companies anymore.

Thanks so much, Avangi.

I meant 'supplier' by 'vendor'. In this case, I want to say it is important suppliers work closely together for some reasons and we are curious about what will be.

LiveinjapanWe will keep an eye on how those vendors can move closely together.

"Those vendors" has a pejorative flavor. "The vendors" is less so.

In the US, we might say, "We'll keep an eye on what the vendors are up to."

"We think they might be up to something" would mean we suspect them of some mischief.

Avangi"We'll keep an eye on what the vendors are up to."

Amazing! Much shorter and much better.