Is Vivien of the Wilds + Wilderness Reclamation a competitive combo?
Let me start by saying this is a really cool idea, and it may be a very fun deck to play in casual format.  Alternatively, you could add this combo to an existing competitive deck that is already leveraging pieces of this combo in other ways (there is at least one suggestion in the comments).

However, this combo on its own is very unlikely to be competitive in standard given how many pieces you need for it to go off.  It's a three card combo at it's heart (Vivien + Wilderness Reclimation + Krasis) that also requires mana acceleration to go off on a reasonable timeframe.  The best-case you present is a turn 7 win, and your whole first 5 turns were spent setting it up, leaving you little buffer to react to your opponent or handle disruption.  On top of that, your plan is very brittle; most combos are vulnerable to control, but yours is also vulnerable to burn and aggro due to many of your key mana sources being creatures and one of your key cards being a planeswalker.

Compare your combo to Carnage Tyrant + Blanchwood Armor, which requires a similar mana base.  This creates a similar sized beater on a similar timeframe.  However, it has several advantages:


Carnage Tyrant is resistant to both control and burn
You can still win with Carnage Tyrant even if you don't draw Blanchwood Armor
It's mono-green, meaning you don't need a complex mana base and can't get color-screwed
You can do other things in your early turns, like Commune With Dinosaurs to help set you up or Vine Mare to defend you from an aggro deck or start an early offense against a control deck.


That is not to say Hydroid Krasis is worse than Carnage Tyrant.  In fact, Krasis is a competitive card in the current standard, but it tends to find its home in control decks rather than combo, especially since card advantage is a big focus of control and Krasis doubles as card advantage and a finisher.

Another thing to notice about decks that use Krasis in the current meta is that (aside from Llanowar Elves) they tend to focus on creatures with the "Explore" ability rather than creatures that tap directly for mana.  While Explore is slower in terms of mana ramp, it is more resilient as it is still relying on land-based mana, and it lets you do a bit of deck filtering/thinning as well.  In your case in particular, you want land mana rather than creature mana because Wilderness Reclamation only untaps lands.