Let me explain 20 inch (406) and 22 inch (451). The numbers in bracket is the figure in millimeters for the diameter of the rim where the bead of the tires rest. Both are classified as 20 inch just like the 16 inch rims where 349 is used for Bromptons and 305 for other models. Both are classified as 16 inch. First you have to consider what terrain are you using your bike? If it sometimes goes a little gravel roads or some slight offroading or into some fields, then 451 is not suitable as the tires are quite limited to slimmer or slicks (botak kind). They are meant for low resistance tires like road bikes. The less resistance compensates the same effort needed for a smaller 406 but off course a 406 with slick tires are much easier to pedal. There are pros and cons, eg.
When we were riding Sekinchan to Sungai Besar, we decided to use the coastal road hugging the beach and this road is like those in rubber estate, offroad! Those on slicks finds it more challenging and has to keep more attention to riding where else those on bigger 406 tires with bigger treads just cruise over it. Then when going through wet or muddy patches, 406 with bigger tires triumps over those on 451s due to the lack of treads on 451 tires. So, like I said, priority must be ascertain before you jump onto the wagon.
As for the choice of bikes, that's totally your choice. Again your priority, if budget is a constraint, then go for the Java. It seems to have everything but if you ride more seriously and join other groups, I can bet you that you will eventually change as you improve. It's not a bad bike, even XDS also copied their design. But Dahon is more expensive for a reason. The MU is a proven frame compared to the Vigor, in fact the dolphin design of the MU is the most copied design by other makes. Java FIT on the other hand, IMO, has disc brakes which have lower performing brakes than the V-brakes of the Dahons (most mechanical disc brakes are anyway), have much wider fold, hard to put it in a bag, shorter wheelbase which makes riding more twitchy (but can be gotten used to) , a wider front fork as it uses 100 mm hubs and much heavier. Dahon Vigor has a conventional chain stay which makes the frame stronger and also the geometry a better ride though. The best is still the Dahon Speed. Even Joshua Hon from Tern claims that before.
I suggest you just buy one on impulse and ride, only then you can understand what it feels like and where your priority lies. Before that, whatever we say here doesn't really mean much. I have on so many instances suggest something but when they head to a local bike shop, they go for another.

It makes it easier to change bike if you go for Dahon because later stages if you want to change, regardless of whether it is a mountain bike, roadbike or better foldie, it's easier to sell.
Once again many thanks for the input !! i think azamu have Speed P8. Alright so forget about Java now.
Speed P8 is even cheaper compare to Vigor and MU. If performance is the concern as i got fren with RB will ride together 95% will be just flat road.
between speed Vigor MU which should i take ? 3 also i like the design lol. Just which one worth for the bucks and easy upgrade later.