The Frenchman looked likely to leave Red Bull's sister team when the young Dutchman was awarded a drive for 2015.
However, with Daniil Kvyat being offered the chance to replace the Ferrari-bound Sebastian Vettel next season, Tost has revealed that no decision has yet been made regarding their driver lineup and spoke highly of Vergne.
He told Formula1.com: "Everything is still open. Nothing is decided yet. Red Bull will make a decision within the next weeks and then we will know whether we will head into the 2015 season with two rookies or with one experienced driver and one rookie.
"It will be close to Abu Dhabi – the week after Abu Dhabi. We have the luxury of time – there is no hurry.
"I am still convinced that Jean-Eric can do a very good job in Formula 1 if he gets a competitive car. He has shown this a couple of times and I personally would like to continue with him – but we have to wait to see what Red Bull decides."
Vergne has competed in 55 grands prix since his debut in 2012.