Lovren has spoken publicly about his wish to move to Liverpool, although Koeman insisted he wanted to keep hold of the player after full-back Luke Shaw, forward Adam Lallana and striker Rickie Lambert all left Southampton this summer.
Vlaar was one of the unlikely stars of the World Cup in Brazil, helping the Netherlands to third place at the tournament with a series of rock-solid displays at the heart of their defence.
The 29-year-old has one year left on the three-year deal he signed at Aston Villa after arriving from Feyenoord in August 2012.
"Ron would definitely be an option," Koeman told Fox Sports. "But I think he won't be cheap right now. Let's just wait and see."
Koeman said he was not very impressed with Lovren's attempts to push through a move to Anfield, with the 25-year-old telling Croatian media that his "head was already at Liverpool".
"A player shouldn't say something like that, it's not very proper," said Koeman, who played for Ajax, PSV Eindhoven and Barcelona in a glittering playing career.
"We just have to wait how it develops. For now, Dejan can still play with us. I don't want to lose him but, if he leaves he will only go for a top price."
Lovren has returned to Southampton for pre-season training.