And the skipper does not want to become a forgotten side who made it to the quarter-final or semi-final as they prepare to host Flackwell Heath in the last-eight on Saturday.
US Portsmouth have already won six ties in the 2020/21 tournament - as many as the club had won in their previous 16 Vase campaigns put together.
Glenn Turnbull's troops laid down a real marker when they triumphed 3-1 at highly fancied Tavistock last weekend - beating a higher division team in the Vase for the fifth time in six matches.
However, Jeffes stressed: 'As good as it is and where we are, we haven’t achieved anything yet and, in my opinion - I honestly believe it - we’re still a couple of games away from achieving anything.
‘No-one remembers the quarter-finalists or the semi-finalists.
‘It’s about not playing the occasion, going out there and proving we should be in the quarter-finals for a reason and getting ourselves into the semi-finals.
'If we lose Saturday, I know how much it’s going to hurt. As much as for me football is about winning - I don’t know more competitive people than myself - I really feel as good as it is we don’t just want to fall over and lose in the quarter-finals.
‘I feel as a club we deserve it, but when you get to this stage you have to earn the right in every game you play.
‘The belief is there, the confidence is there, but you have to keep proving it week after week.
‘You win a game last week but when it gets to this game that means nothing, you’ve got to do it all over again.'
Jeffes admitted trying not to think about Wembley with USP just two wins away from a monumental occasion is hard.
But he is confident their full focus will be on Flackwell Heath when they kick-off their quarter-final tie.
Jeffes said: ‘It’s like an impossible task. You’re trying to be modest and level-headed that we’re a step 6 club, not paying any players or staff, just trying to do what we do. On the flip side, we’re in the quarter-finals of a national competition that, no disrespect to the lads, I know no-one has been to this stage before.
‘On one hand you’re daring to dream, but my job is to try to keep everyone level-headed.
‘The reality is Wembley could be 180 minutes away but we could not turn up Saturday, get turned over and it’s gone a lot quicker than it’s taken to get here.
‘When you’ve got young lads, it’s trying to come away from that distraction.'