anubite:
Ring Target changes the battle chart for certain type combinations of attacks (for example, if the opponent is a Ghost type, it allows Normal- and Fighting-type attacks to be effective against them). It doesn't make attacks work which are ineffective against the opponent for other reasons (for example, if Levitate or Lightningrod makes the attacks ineffective.)
However, a Sawsbuck's type is Normal and Grass, which is not a type combination that causes immunity to Grass-type attacks, so Ring Target has no effect on Grass-type attacks against Sawsbuck.
Sap Sipper triggers when the attack is targeted, so it can trigger only once even if the attack is a multi-hit attack.
References:
My description for Ring Target is:
"Attacks against the holder have normal effectiveness against types that those attacks' types would otherwise be ineffective against. (For example, if the holder's type includes Dark, Psychic-type attacks against it have normal effectiveness against the Dark type.)"
My description for Sap Sipper is:
"Whenever a Grass-type attack ... by a non-bearer targets bearer ... increases bearer's Attack by 1 stage and that attack is ineffective against bearer."
My notes on multi-hit attacks include the following sentence:
"If the attack is ineffective against the opponent, no hits of the attack are done."
EDIT: Corrected.