'loss of' would describe what you lost, in this case vigor

Here are the uses of 'in' as a preposition:

expressing the situation of something that is or appears to be enclosed or surrounded by something else.
"living in Deep River"
synonyms: inside, within, in the middle of; More
expressing motion with the result that something ends up within or surrounded by something else.
"don't put dye in the bathtub"
synonyms: into, inside
"he put a candy in his mouth"
2.
expressing a period of time during which an event takes place or a situation remains the case.
"they met in 1885"
synonyms: during, in the course of, over
"they met in 1921"
3.
expressing the length of time before a future event is expected to take place.
"I'll see you in fifteen minutes"
synonyms: after, at the end of, following; More
4.
(often followed by a noun without a determiner) expressing a state or condition.
"to be in love"
indicating the quality or aspect with respect to which a judgment is made.
"no discernible difference in quality"
5.
expressing inclusion or involvement.
"I read it in a book"
6.
indicating someone's occupation or profession.
"she works in publishing"
7.
indicating the language or medium used.
"say it in Polish"
indicating the key in which a piece of music is written.
"Mozart's Piano Concerto in E flat"
8.
as an integral part of (an activity).
"in planning public expenditure it is better to be prudent"


None fit in this case.

Jim@StratusPrep wrote:'loss of' would describe what you lost, in this case vigor

Here are the uses of 'in' as a preposition:

expressing the situation of something that is or appears to be enclosed or surrounded by something else.
"living in Deep River"
synonyms: inside, within, in the middle of; More
expressing motion with the result that something ends up within or surrounded by something else.
"don't put dye in the bathtub"
synonyms: into, inside
"he put a candy in his mouth"
2.
expressing a period of time during which an event takes place or a situation remains the case.
"they met in 1885"
synonyms: during, in the course of, over
"they met in 1921"
3.
expressing the length of time before a future event is expected to take place.
"I'll see you in fifteen minutes"
synonyms: after, at the end of, following; More
4.
(often followed by a noun without a determiner) expressing a state or condition.
"to be in love"
indicating the quality or aspect with respect to which a judgment is made.
"no discernible difference in quality"
5.
expressing inclusion or involvement.
"I read it in a book"
6.
indicating someone's occupation or profession.
"she works in publishing"
7.
indicating the language or medium used.
"say it in Polish"
indicating the key in which a piece of music is written.
"Mozart's Piano Concerto in E flat"
8.
as an integral part of (an activity).
"in planning public expenditure it is better to be prudent"


None fit in this case.

so we can say confidently that loss in vigor is not idiomatic in usage.
right!

Spot on. prepositions can be tricky because the meanings are subtly different and often misused.