[R] var:covariance matrix from glm using logit function

[Christopher Dennis]

  I want to predict a set of proportions from a linear regression using glm.
The model includes a logit link. However I want to extract the 
variance:covariance matrix among the predicted proportions rather than 
on the logit scale.
Is there a way to do this using Vcov or a similar package in R?
Thanks
Chris