If a company goes bankrupt, is a fully vested 401k account in jeopardy?

Depends on how the money was invested, Normally the money is piad into a investment company and invested in funds or stock.

But there are possible to have loss in some cases

If a company goes bankrupt, is a fully vested 401k account in jeopardy?

No - monies invested in a 401(k) are not part of your employer's assets, so your 401(k) should be in jeopardy. Of course if you invested in the company stock within your 401(k) then the value of that portion of your investment may be wiped out.