Catlin is expecting a tough game against their hosts who play one level below in Division One of the Western League.
The Royals boss is keen to see his side return to winning ways after their disappointing 1-0 home derby defeat to Fareham Town.
'I believe if we get through then it is the furthest the club has ever been in the Vase,' said Catlin.
'Though we will start the game as favourites, that will count for nothing if we don't improve on our midweek display.
'It was the worst we have played all season against Fareham and the first time we haven't scored in a game.
'We let the occasion get to us and looked nervous.
'They unsettled us and we got sucked into knocking the ball long.
'Warminster will see it as a free hit and will try to do the same.
'They have already beaten Alresford in the competition so can't be taken lightly.
'It is up to us to show a bit more quality on the ball.'
The Royals match-winner in the last round at Cullompton Rangers, Jason Parish, comes back into the side.
After picking up a knock at Hamble a fortnight ago, he wasn't risked in midweek.
'Jason is big, strong and able to get behind defenders,' said Catlin.
'The signs are that he and Dan Wooden can form a good partnership up front and can get the goals we need.'
Portchester have previously reached the third round twice - in 2014/15 and 2015/16. They were rewarded with home draws in both seasons but lost to Blackfield and Tunbridge respectively.
Baffins Milton Rovers make their bow in this season’s Vase against Western League Premier outfit Bradford Town at the PMC Stadium.
Rovers had a bye in the earlier rounds after reaching the last 32 in 2018/19, including a win at Bradford.
Baffins boss Steve Leigh believes the Wiltshire side will be stronger this time around.
'Bradford are flying and top of their league,' said Leigh.
'This will be a very tough tie.
'They have already talked about exacting revenge for last season.
'We have to forget all about the past and focus on this one-off game.
'It is up to us to turn up and show that we are up for it.
'If we don't then we will be out of the Vase.
'The only pressure on my players is to go and give 100 per cent.
'If they do that, whatever the result, I will pat them on the back.'
There is a fitness doubt about midfielder Georgie Way.
The recent return of Blu Boam after a long lay off, however, has been a massive boost.
Petersfield Town entertain Western League Premier Division Roman Glass St George at Love Lane (3pm).