I have bad experience with slider so, might be bias here if u can tolerate with not so good camera of X2, IMO, it is better than all.
*to be honest, zylo is quite solid there. but slider always got a ribbon problems. and, it still use SE fastport (i hate it to the max)
X8! vivaz is nearly dead.. no more support from SE. it's camera sure is nice, but, it will remain as that. no bug will be fixed.
X8 as android always wins on apps, including games. the screen size is 0.2" less than vivaz. but it is enough to be honest.
strongly suggest u to put side vivaz. unless the HD record can mask away the lack of support (dead) and very soft loudspeaker
apart from these two, if u can up your budget up to 899, can get optimus one. it is better IMO. with android 2.2

is tat vivaz so that bad?is x8 is possible to get his picture zoom one day?after upgrade to android 2.1?

suggest u get lg optimus one if u can increase ur budget
if can't then try csl mi300
both of them got android 2.2 official(not sure about csl mi300)
and x8 not say no good,but just got official android 2.1 oni,no more official update from SE ~

erm.ok then.