How to get vertices of a polytope given by equalities and inequalities?

Question

I have a polytope given by some equalities and inequalities, how can I get all the vertices of the polytope? If there's a way to deal with the one given by inequalities only, how can I reduce the equalities and inequalities to inequalities only?

--Updated 2013.12.11--

m=SparseArray[Automatic,{28,56},0,{1,{{0,6,12,18,24,39,50,61,67,73,79,94,105,116,122,128,143,154,165,185,205,234,257,268,279,302,313,324,335},{{1},{2},{3},{4},{5},{6},{1},{7},{8},{9},{10},{11},{2},{7},{12},{13},{14},{15},{3},{8},{12},{16},{17},{18},{1},{2},{3},{4},{5},{7},{8},{9},{10},{12},{13},{14},{16},{17},{19},{1},{2},{3},{5},{7},{8},{10},{12},{14},{17},{20},{1},{2},{3},{4},{7},{8},{9},{12},{13},{16},{21},{1},{22},{23},{24},{25},{26},{2},{22},{27},{28},{29},{30},{3},{23},{27},{31},{32},{33},{1},{2},{3},{4},{5},{22},{23},{24},{25},{27},{28},{29},{31},{32},{34},{1},{2},{3},{5},{22},{23},{25},{27},{29},{32},{35},{1},{2},{3},{4},{22},{23},{24},{27},{28},{31},{36},{7},{22},{37},{38},{39},{40},{8},{23},{37},{41},{42},{43},{1},{7},{8},{9},{10},{22},{23},{24},{25},{37},{38},{39},{41},{42},{44},{1},{7},{8},{10},{22},{23},{25},{37},{39},{42},{45},{1},{7},{8},{9},{22},{23},{24},{37},{38},{41},{46},{1},{2},{3},{4},{7},{8},{9},{12},{13},{16},{22},{23},{24},{27},{28},{31},{37},{38},{41},{47},{1},{2},{3},{5},{7},{8},{10},{12},{14},{17},{22},{23},{25},{27},{29},{32},{37},{39},{42},{48},{1},{2},{3},{4},{5},{7},{8},{9},{10},{12},{13},{14},{16},{17},{22},{23},{24},{25},{27},{28},{29},{31},{32},{37},{38},{39},{41},{42},{49},{1},{2},{3},{4},{5},{7},{8},{9},{10},{12},{16},{17},{22},{23},{24},{25},{27},{31},{32},{37},{41},{42},{50},{1},{3},{5},{8},{10},{17},{23},{25},{32},{42},{51},{1},{3},{4},{8},{9},{16},{23},{24},{31},{41},{52},{1},{2},{3},{4},{5},{7},{8},{9},{10},{12},{13},{14},{22},{23},{24},{25},{27},{28},{29},{37},{38},{39},{53},{1},{2},{5},{7},{10},{14},{22},{25},{29},{39},{54},{1},{2},{4},{7},{9},{13},{22},{24},{28},{38},{55},{1},{2},{3},{7},{8},{12},{22},{23},{27},{37},{56}}},{1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,-2,-2,-2,-1,-1,-2,-2,-1,-1,-1,-1,-1,-1,-1,-2,-2,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,-2,-1,-2,-1,-1,-1,-2,-1,-1,-1,-1,-1,-1,-2,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,-2,-2,-1,-1,-1,-2,-1,-1,-1,-1,-1,-1,-2,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}}]; b={-(2/7),-(2/7),-(2/7),-(2/7),-(5/7),3/7,3/7,-(2/7),-(2/7),-(2/7),-(5/7),3/7,3/7,-(2/7),-(2/7),-(5/7),3/7,3/7,-(20/21),-(20/21),34/21,25/21,-(11/21),-(11/21),25/21,-(11/21),-(11/21),-(11/21)};

The equalities are m.x==b and the inequalities are x>=0

Answer 1

To put Daniel's comment into code:

x = Array[C, Length@First@m];
constr = First@Solve[m.x == b]
(*
  {C[6] -> -(2/7) - C[1] - C[2] - C[3] - C[4] - C[5], <<26>>,
   C[56] -> -(11/21)-C[1]-C[2]-C[3]-C[7]-C[8]-C[12]-C[22]-C[23]-C[27]-C[37]}
*)

Reduce[Thread[x >= 0] /. constr, Variables[x /. constr]]
(*  False  *)

Easy, fast, and disappointing. The polytope is an empty set.

The closest point to the origin consists of the vector with coordinates all equal to -1/21:

PseudoInverse[m].b
(*  {-(1/21), -(1/21), <<25>>, -(1/21)}  *)

This vector is normal to the affine subspace m.x == b, so the subspace does not intersect the positive orthant.