Lowest federal result I can find is a Tory winning 22% of the vote in Verdun in 1935. They managed to win thanks to the CCF and Labour splitting the left wing vote almost evenly. More recently the Bloc won Gatineau on 29 % of the vote in 2008.
Brad Trost (C) won with 26.7% in Saskatoon-Humboldt in 2004. (4-way split with 4 candidates getting over 20%, including Con-turned-indy incumbent Jim Pankiw.)
In 2015, the following sub-30% victors, all in Quebec thanks to Lib/NDP/Con/Bloc splitting it 4 ways:
Bernard Genereux (C), Montmagny et al, 28.99%
Karine Trudel (N), Jonquiere, 29.19%
Jean-Yves Duclos (L), Quebec, 28.90%
Brigitte Sansoucy (N), St-Hyacinthe-Bagot, 28.65%
Xavier Barsalou-Duval (B), Pierre-Boucher et al, 28.64%