Is there any positive or negative about this?
Yes, it will affect you. Your choice of funds in which to invest your money will change. Will it be negative or positive? I can't say, but if your company is changing, you have no choice. I am guessing that Vanguard's fees are a little lower, but if you are restricted to choosing funds from only the Vanguard family of funds, your investment potential will be less than what you could realize with choices from the broad spectrum of funds out there, offered by multiple companies. But if you are restricted to Vanguard only, you'll probably do better than you would if Guardian has similar restrictions.
Vanguard is one of the best mutual fund companies and the costs are very low.