Sigma schedule last time i hear is 2017 and nothing change yet
And about why VN choose Gepard with minimum options, the cause start from BPS-500 scandal, after that there are one rule in VN for purchase waepon, that is we just chose the type which is prove the power in other country, not the new concept with unclear capability, that why VN choose Gepard instead of Tiger class, and one more thing you must understand that at the time VN purchase gerpad, there are only 3 gepards in Russian Navy, but their desgin also defferent with VN, after that gerpad type is started to improve, and some other class appear, that is normal. And we cannot compare 2 first gerpad with recent model is quite understandable because they did a lot of upgrade recently
for example:
Russian Navy gerpad: even don't have heli deck ( check on
Dagestan ship)
and Vietnam
and 3rd and 4th even equip with toperdo system ( Parket- NK)
and gepard with new design( introduce in 2014 ??)
you can see there are a lot of upgrade, with 2 Palma and may be Shtil-SAM VLS
Talwar is a new design, but i think the price cannot be about 250 mil $, it must be about 400-500 mil 4 for a 4000tons ship, when Vietnam start to purchase 4000 ton ship, it can be an option, but now is not yet
last time i check, Indian navt pay 3 bil $ for 3 talwars
(may be with technology transfer)