Aspeaker who argues, "sammy can vouch for me. you can trust sammy because i think he is a good person," is using which fallacy to advance his/her claim?

The fallacy used to his/her claim is by begging the questions.

Begging the questions (petitio principii) is where the conclusion is assumed in one of the premises. This is a logical fallacy in which the writer or the speaker assumes the statement under examination to be true. In simple explanation, to beg the question is to assume something that you have no right to assume.