The reforms envisage letting heads mix the vocational and academic study at existing schools from the age of 14. But the Tories want to see separate "technical" senior schools, as in many European countries.
In a speech to the Social Market Foundation on Tuesday, shadow education secretary Damian Green said: "We all agree that vocational education in Britain needs to be done better and started earlier. The question is how to achieve this.
"The reforms outlined in the 14-19 paper are timid and largely irrelevant to the twin essential aims of re-skilling our workforce and providing a rewarding educational experience for the non-academic.
"Instead we should be allowing many more real technical schools to develop as part of the move into a post-comprehensive era. At these schools, those whose bent is for vocational education should be allowed to start serious work in their chosen fields from the age of 13."
He said this system worked well in countries like Holland, where he visited two schools this week with party leader Iain Duncan Smith.
Mr Green said too many young people were allowed to drift, without any activity at which they could excel or which seemed relevant to the life they wanted to lead. "These are the youngsters most likely to become truants. We need to give them a good chance in life."
His comments came on the same day that Mr Duncan Smith unveiled plans to "rescue" the 30,000 youngsters who leave school without qualifications, by allowing them to focus on vocational study.