NORTH
 J9654
 J98
 Q
 AKJ3
 
 QT63
 AK74
 Q9862
  A
 AK752
 JT62
 T54
  KQT8732
 4
 9853
 7
Dealer: North    Vul: EW   MP Scoring

West

5
Pass

North
1
Pass
Pass

East
2
Pass
Pass

South
4
5

 Forcing Auction? 

What constitutes a forcing auction?
Whatever the meaning of South's 4, West's Vulnerable vs Not-Vulnerable 5 should establish a forcing auction for East - West, shouldn't it?
Perhaps it's easier to have just one standard for a forcing auction, regardless of vulnerability than it is to suggest that:
Vul vs Not Vul IS a forcing auction
Not Vul vs Vul is NOT a forcing auction
and the other two equal vulnerability situations: Vul vs Vul, and None Vul, might be in dispute.

Certainly vulnerability is to be considered, but to introduce such standards for many pairs is a bit much.

How would your partnership decide on the Forcing or Not issue here?

West was flabberghasted when East passed out 5.
West's PASS over 5 suggested 1st round control of spade (alternatively 4+ small spades) PLUS interest in a heart slam. Was a heart slam so impossible?
If East's A had been the A and East had additionally held the two points more that the K traded for the J would have been, 6 was THE place to be, and why not then 7?

With 4 points in spades on defence, East should have doubled.

If East West had, however, been Not Vul vs. Vulnerable North South, perhaps a weak sacrificial 5 more commends itself on the basis of probability of occurence and weighting of Matchpoint scorings.