As a native speaker I assure you that 請 is pronounced differently in context, and that both readings are perfectly Cantonese.
For a more clear example, see 平: 大平賣 ("vernacular" reading; peng4) (lit. big cheap sale; i.e. sale) vs 平面 ("literary" reading; ping4) (lit. flat surface; i.e. surface). peng4 is the "vernacular" reading but used exclusively for meaning cheap. ping4 is "literary" but used everywhere else. "vernacular" versus "literary" is a linguistic classification, but do not necessarily represent either being more common than the other reading. Both readings exist.
For a more clear example, see 平: 大平賣 ("vernacular" reading; peng4) (lit. big cheap sale; i.e. sale) vs 平面 ("literary" reading; ping4) (lit. flat surface; i.e. surface). peng4 is the "vernacular" reading but used exclusively for meaning cheap. ping4 is "literary" but used everywhere else. "vernacular" versus "literary" is a linguistic classification, but do not necessarily represent either being more common than the other reading. Both readings exist.