Consider G as operating on E on the left by the mapping (g,x)↦π(g)x. Let b∈E and φ(b) be its orbital mapping. Let Tb(E) be canonically identified with E. For all t∈L(G),
(L(π)t)(b)=⟨t,φ(b)⟩=φ(b)∗t=t∗εb.
In particular, the vector field defined by t on E is the field b↦(L(π)t)(b).
L(π)t=⟨t,π⟩ (no. 9, Proposition 34). As the mapping A↦Ab of L(E) into E is continuous and linear, it follows that
(L(π)t)(b)=⟨t,g↦π(g)b⟩=⟨t,IdE∘φ(b)⟩=⟨φ(b)∗t,IdE⟩(Diff. & Anal. Man., R, 13.2.3)=φ(b)∗t.
Finally, φ(b)∗t=t∗εb (no. 3, Proposition 14 (ii)).
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.