Morten Thorsby came close after 10 minutes but the midfielder's header was straight at Lamprou as the away side continued to be on the front foot.
However, it would be Willem II to score first through Ajax loanee Richairo Zivkovic, who connected with a Funso Ojo cross-field pass and the Dutchman lifted his first-time volley over the onrushing Erwin Mulder.
Heerenveen pulled themselves level in the second half as Henk Veerman fired home from close range but Lucas Andersen put the home side back in front after 70 minutes with a good finish.
Slagveer then brought the visitors back level after he connected with Stefano Marzo's cross to tap home from close range.
Willem had the chance to win with three minutes remaining after Robert Braber was fouled by Caner Cavlan in the box but Stijn Wuytens's penalty was saved by Mulder before it cannoned off the post.
The draw lifts Willem II outside of the relegation places in the Eredivisie on goal difference while Heerenveen remain a point above them in 14th.