Back to "basics" then.
You should always start troubleshooting from any error messages you can see. (They are your best "clue"; and why they exist at all.) Reporting such messages accurately is an essential first step.
Anytime the game "Crashes to the Desktop" (CTD) it's typically going to generate an error message in the Windows Eventlog because the game engine was not able to predictably handle the error. Please see the "Windows Error Messages" section of the wiki "How to read most Bethesda game error logs" article.
Logs generated by the game itself (not Windows) are found in the game folder, where the "FalloutNV.EXE" file is located. They generally have the ".log" file extension, or a filename with "_log" in the name and a ".txt" extension.
A "CTD" is a "fatal" error, meaning the program/game did not know how to handle the error, nor how to proceed from that point; and shutdown to let the Windows Operating System (OS) take over again from the "desktop". The only thing worse is the so-called "Blue Screen of Death (BSoD)", which means the OS itself encountered a similar fatal error and entire computer has to be "rebooted" ("reset" switch or "power off-for-10-seconds-and-on-again") to return to function. DO NOT attempt to resume the game again from the point of a CTD, as your system memory is in an unpredictable state and make cause "save game" corruption even if it does get past the point where it crashed before. Reboot first.
"NVAC - New Vegas AntiCrash" error log entries are described on the "Description" page. This an important log for "in-game" errors. Some are intended to assist mod authors; while others are there to help "you, the player" trying to resolve a problem. You are interested in the entries which are NOT automatically handled successfully by NVAC, and records which do NOT begin with "00" (which are in the vanilla game files and not typically going to be fixable by yourself). Key entries to look for are the following "letter codes":
* "e" entries are Exceptions handled specifically by NVAC's targeted code changes
* "h" entries are exceptions identified Heuristically by NVAC's "global" exception handler, sometimes handled unsuccessfully.
* "m" entries are "Middle of nowhere" exceptions, sometimes handled unsuccessfully.
* "ñ" entries are Null instruction exceptions, sometimes handled unsuccessfully.
* "n" entries are Null pointer call exceptions, sometimes handled unsuccessfully.
* "o" entries are stack Overflow exceptions, usually handled unsuccessfully.
* "q" entries are program self-termination information; these are effectively crashes.
* "u" entries are Unhandled exceptions reported from the Unhandled Exception Filter; these are crashes (and the NVAC author wants to hear about them).
* "v" entries are unhandled exceptions reported by NVAC's "global" exception handler; these are crashes.
* "w" entries are informative, relaying exception information as it passes through New Vegas's final exception handler.
* "x" entries are critical errors in Windows exception data; these entries should never occur.
* ":" entries are output from OutputDebugString calls.
* ";" entries are output from the game's own error checking.
Other codes may be of interest, but are not things the player is likely to be able to resolve, They are useful for informing the author of the mod to aid them in fixing their plugin. In particular, pass along the:
* "_" entries are informative, relaying that NVAC was loaded and the base address of various DLLs
* "!" entries are informative, relaying human-readable text for the user entries.
Any error codes found in the log but not listed on the mod "Description" page should be reported to the NVAC comments page.
'Expression Error: SYNTAX' is not uncommon, even in vanilla without any mods loaded. These are basically "typos". Such errors are reported by NVAC but ignored. Those lines of script code do not get executed, but also are not "fatal" to the game. They are the sort of thing mods like "Yukichigai Unofficial Patch - YUP", "JIP LN NVSE", "JohnnyGuitar NVSE", "lStewieAl's Tweaks" etc. are designed to fix or supplement if possible.
"Windows Exception code: 0xc0000005". The "Faulting Application" is often the victim of another program rather than the actual culprit, making it virtually impossible for the user to trace the source. In at least one instance it appears to be caused by VWD/LOD files that had not been regenerated (when caught by NVAC as "DISABLED _LOD_ERR"). Any recent update since the last time it was working correctly is suspect, as are viruses even if you have anti-virus (AV) running. Run AV from multiple sources when checking for infections. There are a number of free online scanners on the internet (though they are all going to try to sell you their services after the first time).
You may need to exclude FNV from "Data Execution Protection (DEP)" and configure " Windows Defender Exploit Guard (WDEG)" (or Microsoft's "Enhanced Mitigation Experience Toolkit (EMET)" for Win7). Please see "Issue - NVSE - fails to load after update KB4058043 to Win10 FCU v1709" under the 'Solutions to Starting the game problems' section of the wiki "Fallout NV Mod Conflict Troubleshooting" article.
Try turning off any unnecessary background processes, such as "anti-virus" or "anti-malware" programs (unneeded if you are running your game in "offline" mode once you have scanned since the last connection), multiplayer "overlays" like "OverWolf" (this is single-player after all), the Steam overlay, etc., at least on a temporary basis to see if they are interfering. There are also tools (often called "Game Boosters") to help you determine "unnecessary when offline" Windows services you can temporarily shut down.
It is also possible that any recent change to your mods (such as updates) or your existing "load order" (such as additions or removals) has caused a corruption of the current save game file. Try going back to a save game from before the problem started. Often starting a "new game" with a vanilla test character can help confirm if the problem lies in the current load order.